---
title: "Build your Go image"
keywords: containers, images, go, golang, dockerfiles, coding, build, push, run
description: Learn how to build your first Docker image by writing a Dockerfile
redirect_from:
- /get-started/golang/build-images/
---

{% include_relative nav.html selected="1" %}

## Prerequisites

* Some understanding of Go and its toolchain. This is not a tutorial on Go. If
  you are new to the language, the [Go website](https://golang.org/){: target="_blank" rel="noopener" class="_"}
  is a good starting point, so go (pun intended) check it out.
* Some awareness of basic Docker concepts. If unsure, work through the orientation
  and setup in Get started [Part 1](../../get-started/index.md).

## Overview

Now that we have a good overview of containers and the Docker platform, let’s
take a look at building our first image. An image includes everything you need
to run an application – the code or binary, runtime, dependencies, and any other
file system objects required.

To complete this tutorial, you need the following:

- Go version 1.16 or later. You might want to [download and install Go](https://golang.org/dl/) first.
- Docker running locally. Follow the instructions to [download and install Docker](../../desktop/index.md).
- An IDE or a text editor to edit files. We recommend using [Visual Studio Code](https://code.visualstudio.com/){: target="_blank" rel="noopener" class="_"}.

## Meet the example application

To avoid losing focus on Docker's features, the sample application is a minimal
HTTP server that has only three features:

* It responds with a text message containing a heart symbol ("<3") on requests to `/`.
* It responds with `{"Status" : "OK"}` JSON to the health check request on requests to `/ping`.
* The port it listens on is configurable using the environment variable `HTTP_PORT`. The default value is `8080`.

Thus, it somewhat mimics enough basic properties of a REST microservice to be
useful for our learning of Docker.

The source code for the application is in the [github.com/olliefr/docker-gs-ping](https://github.com/olliefr/docker-gs-ping){: target="_blank" rel="noopener" class="_"}
GitHub repository. Please feel free to clone or fork it.

For our present study, we clone it to our local machine:

```console
$ git clone https://github.com/olliefr/docker-gs-ping
```

The application's `main.go` file is fairly straightforward, if you are familiar
with Go:

{% raw %}
```go
package main

import (
	"net/http"
	"os"

	"github.com/labstack/echo/v4"
	"github.com/labstack/echo/v4/middleware"
)

func main() {

	e := echo.New()

	e.Use(middleware.Logger())
	e.Use(middleware.Recover())

	e.GET("/", func(c echo.Context) error {
		return c.HTML(http.StatusOK, "Hello, Docker! <3")
	})

	e.GET("/ping", func(c echo.Context) error {
		return c.JSON(http.StatusOK, struct{ Status string }{Status: "OK"})
	})

	httpPort := os.Getenv("HTTP_PORT")
	if httpPort == "" {
		httpPort = "8080"
	}

	e.Logger.Fatal(e.Start(":" + httpPort))
}
```
{% endraw %}

## Smoke test the application

Let’s start our application and make sure it’s running properly. Open your
terminal and navigate to the directory into which you cloned the project's repo.
From now on, we'll refer to this directory as the **project directory**.

```console
$ go run main.go
```

This should compile and start the server as a foreground application, outputting
the banner, as illustrated in the next figure.

```
   ____    __
  / __/___/ /  ___
 / _// __/ _ \/ _ \
/___/\__/_//_/\___/ v4.2.2
High performance, minimalist Go web framework
https://echo.labstack.com
____________________________________O/_______
                                    O\
⇨ http server started on [::]:8080
```

Let's run a quick _smoke test_ on the application. **In a new terminal**, run a
request using `curl`. Alternatively, you can use your favourite web browser as
well.

```console
$ curl http://localhost:8080/
Hello, Docker! <3
```

So, the application responds with a greeting, just as the first business
requirement says it should. Great.

Having established that the server is running and is accessible, let's proceed
to "dockerizing" it.

## Create a Dockerfile for the application

{% include guides/create-dockerfile.md %}

Next, we need to add a line in our Dockerfile that tells Docker what base image
we would like to use for our application.

```dockerfile
# syntax=docker/dockerfile:1

FROM golang:1.16-alpine
```

Docker images can be inherited from other images. Therefore, instead of creating
our own base image, we’ll use the official Go image that already has all the tools
and packages to compile and run a Go application. You can think of this in the
same way you would think about class inheritance in object oriented programming
or functional composition in functional programming.

When we have used that `FROM` command, we told Docker to include in our image all the functionality from the `golang:1.16-alpine` image. All of our consequent commands would build on top of that "base" image.

> **Note**
>
> If you want to learn more about creating your own base images, see [creating base images](../../develop/develop-images/baseimages.md) section of the guide.

To make things easier when running the rest of our commands, let’s create a
directory _inside_ the image that we are building. This also instructs Docker
to use this directory as the default _destination_ for all subsequent commands.
This way we do not have to type out full file paths but can use relative paths
based on this directory.

```dockerfile
WORKDIR /app
```

Usually the very first thing you do once you’ve downloaded a project written in
Go is to install the modules necessary to compile it.

But before we can run `go mod download` inside our image, we need to get our
`go.mod` and `go.sum` files copied into it. We use the `COPY` command to do this. 

In its simplest form, the `COPY` command takes two parameters. The first
parameter tells Docker what files you want to copy into the image. The last
parameter tells Docker where you want that file to be copied to. 

We’ll copy the `go.mod` and `go.sum` file into our project directory `/app` which,
owing to our use of `WORKDIR`, is the current directory (`.`) inside the image.

```dockerfile
COPY go.mod ./
COPY go.sum ./
```

Now that we have the module files inside the Docker image that we are building,
we can use the `RUN` command to execute the command `go mod download` there as
well. This works exactly the same as if we were running `go` locally on our
machine, but this time these Go modules will be installed into a directory
inside the image.

```dockerfile
RUN go mod download
```

At this point, we have an image that is based on Go environment version 1.16
(or a later minor version, since we had specified `1.16` as our tag in the
`FROM` command) and we have installed our dependencies. 

The next thing we need to do is to copy our source code into the image. We’ll
use the `COPY` command just like we did with our module files before.

```dockerfile
COPY *.go ./
```

This `COPY` command uses a wildcard to copy all files with `.go` extension
located in the current directory on the host (the directory where the `Dockerfile`
is located) into the current directory inside the image. 

Now, we would like to compile our application. To that end, we use the familiar
`RUN` command:

```dockerfile
RUN go build -o /docker-gs-ping
```

This should be familiar. The result of that command will be a static application
binary named `docker-gs-ping` and located in the root of the filesystem of the
image that we are building. We could have put the binary into any other place we
desire inside that image, the root directory has no special meaning in this
regard. It's just convenient to use it to keep the file paths short for improved
readability.

Now, all that is left to do is to tell Docker what command to execute when our
image is used to start a container. 

We do this with the `CMD` command:

```dockerfile
CMD [ "/docker-gs-ping" ]
```

Here's the complete `Dockerfile`:

```dockerfile
# syntax=docker/dockerfile:1

FROM golang:1.16-alpine

WORKDIR /app

COPY go.mod ./
COPY go.sum ./
RUN go mod download

COPY *.go ./

RUN go build -o /docker-gs-ping

EXPOSE 8080

CMD [ "/docker-gs-ping" ]
```

The `Dockerfile` may also contain _comments_. They always begin with a `#` symbol,
and must be at the beginning of a line. Comments are there for your convenience
to allow documenting your `Dockerfile`. Dockerfile _directives_, such as the
`syntax` directive we added, must always be at the very top of the `Dockerfile`,
so when adding comments, make sure they are after those directives:

```dockerfile
# syntax=docker/dockerfile:1

# Alpine is chosen for its small footprint
# compared to Ubuntu
FROM golang:1.16-alpine

WORKDIR /app

# Download necessary Go modules
COPY go.mod ./
COPY go.sum ./
RUN go mod download

# ... the rest of the Dockerfile is ...
# ...   omitted from this example   ...
```

## Build the image

Now that we've created our `Dockerfile`, let’s build an image from it. The
`docker build` command creates Docker images from the `Dockerfile` and a "context".
A build _context_ is the set of files located in the specified path or URL. The
Docker build process can access any of the files located in the context.

The build command optionally takes a `--tag` flag. This flag is used to label
the image with a string value, which is easy for humans to read and recognise.
If you do not pass a `--tag`, Docker will use `latest` as the default value.

Let's build our first Docker image!

```console
$ docker build --tag docker-gs-ping .

[+] Building 3.6s (12/12) FINISHED
 => [internal] load build definition from Dockerfile                                      0.1s
 => => transferring dockerfile: 38B                                                       0.0s
 => [internal] load .dockerignore                                                         0.1s
 => => transferring context: 2B                                                           0.0s
 => [internal] load metadata for docker.io/library/golang:1.16-alpine                     3.0s
 => [1/7] FROM docker.io/library/golang:1.16-alpine@sha256:49c07aa83790aca732250c2258b59  0.0s
 => => resolve docker.io/library/golang:1.16-alpine@sha256:49c07aa83790aca732250c2258b59  0.0s
 => [internal] load build context                                                         0.1s
 => => transferring context: 114B                                                         0.0s
 => CACHED [2/7] WORKDIR /app                                                             0.0s
 => CACHED [3/7] COPY go.mod .                                                            0.0s
 => CACHED [4/7] COPY go.sum .                                                            0.0s
 => CACHED [5/7] RUN go mod download                                                      0.0s
 => CACHED [6/7] COPY *.go .                                                              0.0s
 => CACHED [7/7] RUN go build -o /docker-gs-ping                                          0.0s
 => exporting to image                                                                    0.1s
 => => exporting layers                                                                   0.0s
 => => writing image sha256:336a3f164d0f079f2e42cd1d38f24ab9110d47d481f1db7f2a0b0d2859ec  0.0s
 => => naming to docker.io/library/docker-gs-ping                                         0.0s

Use 'docker scan' to run Snyk tests against images to find vulnerabilities and learn how to fix them
```

Your exact output will vary, but provided there aren't any errors, you should
see the `FINISHED` line in the build output. This means Docker has successfully
built our image and assigned a `docker-gs-ping` tag to it.

## View local images

To see the list of images we have on our local machine, we have two options. One
is to use the CLI and the other is to use [Docker Desktop](../../desktop/index.md).
Since we are currently working in the terminal, let’s take a look at listing
images with the CLI.

To list images, run the `docker image ls`command (or the `docker images` shorthand):

```console
$ docker image ls

REPOSITORY       TAG       IMAGE ID       CREATED          SIZE
docker-gs-ping   latest    336a3f164d0f   39 minutes ago   540MB
postgres         13.2      c5ec7353d87d   7 weeks ago      314MB
```

Your exact output may vary, but you should see `docker-gs-ping` image with the
`latest` tag.

## Tag images

An image name is made up of slash-separated name components. Name components may
contain lowercase letters, digits and separators. A separator is defined as a
period, one or two underscores, or one or more dashes. A name component may not
start or end with a separator.

An image is made up of a manifest and a list of layers. In simple terms, a “tag”
points to a combination of these artifacts. You can have multiple tags for the
image and, in fact, most images have multiple tags. Let’s create a second tag
for the image we had built and take a look at its layers.

Use the `docker image tag` (or `docker tag` shorthand) command to create a new
tag for our image. This command takes two arguments; the first argument is the
"source" image, and the second is the new tag to create. The following command
creates a new `docker-gs-ping:v1.0` tag for the `docker-gs-ping:latest` we built
above:

```console
$ docker image tag docker-gs-ping:latest docker-gs-ping:v1.0
```

The Docker `tag` command creates a new tag for the image. It does not create a
new image. The tag points to the same image and is just another way to reference
the image.

Now run the `docker image ls` command again to see the updated list of local
images:

```console
$ docker image ls

REPOSITORY       TAG       IMAGE ID       CREATED          SIZE
docker-gs-ping   latest    336a3f164d0f   43 minutes ago   540MB
docker-gs-ping   v1.0      336a3f164d0f   43 minutes ago   540MB
postgres         13.2      c5ec7353d87d   7 weeks ago      314MB
```

You can see that we have two images that start with `docker-gs-ping`. We know
they are the same image because if you look at the `IMAGE ID` column, you can
see that the values are the same for the two images. This value is a unique
identifier Docker uses internally to identify the image.

Let’s remove the tag that we had just created. To do this, we’ll use the
`docker image rm` command, or the shorthand `docker rmi` (which stands for
"remove image"):

```console
$ docker image rm docker-gs-ping:v1.0
Untagged: docker-gs-ping:v1.0
```

Notice that the response from Docker tells us that the image has not been removed but only "untagged". Verify this by running the images command:

```console
$ docker image ls

REPOSITORY       TAG       IMAGE ID       CREATED          SIZE
docker-gs-ping   latest    336a3f164d0f   45 minutes ago   540MB
postgres         13.2      c5ec7353d87d   7 weeks ago      314MB
```

The tag `v1.0` has been removed but we still have the `docker-gs-ping:latest`
tag available on our machine, so the image is there.

## Multi-stage builds

You may have noticed that our `docker-gs-ping` image stands at 540MB, which you
may think is a lot. You may also be wondering whether our dockerized application
still needs the full suite of Go tools, including the compiler, after the
application binary had been compiled.

These are legit concerns. Both can be solved by using _multi-stage builds_. The
following example is provided with little explanation because this would derail
us from our current concerns, but please feel free to explore on your own later.
The main idea is that we use one image to produce some artifacts, which are then
placed into another, much smaller image, containing only the parts necessary for
running the artifacts that we'd built.

The `Dockerfile.multistage` in the sample application's repo has the following
content:

{% raw %}
```dockerfile
# syntax=docker/dockerfile:1

## Build
FROM golang:1.16-buster AS build

WORKDIR /app

COPY go.mod ./
COPY go.sum ./
RUN go mod download

COPY *.go ./

RUN go build -o /docker-gs-ping

## Deploy
FROM gcr.io/distroless/base-debian10

WORKDIR /

COPY --from=build /docker-gs-ping /docker-gs-ping

EXPOSE 8080

USER nonroot:nonroot

ENTRYPOINT ["/docker-gs-ping"]
```
{% endraw %}

Since we have two dockerfiles now, we have to tell Docker that we want to build
using our new Dockerfile. We also tag the new image with `multistage` but this
word has no special meaning, we only do so that we could compare this new image
to the one we've built previously, that is the one we tagged with `latest`:

```console
$ docker build -t docker-gs-ping:multistage -f Dockerfile.multistage .
```

Comparing the sizes of `docker-gs-ping:multistage` and `docker-gs-ping:latest`
we see an order-of-magnitude difference!

```console
$ docker image ls

REPOSITORY       TAG          IMAGE ID       CREATED              SIZE
docker-gs-ping   multistage   e3fdde09f172   About a minute ago   27.1MB
docker-gs-ping   latest       336a3f164d0f   About an hour ago    540MB
```

This is due to the fact that the ["distroless" base image](https://github.com/GoogleContainerTools/distroless){:target="_blank" rel="noopener" class="_"}
that we have used to deploy our Go application is very barebones and is meant
for lean deployments of static binaries.

For more information on multi-stage builds, please feel free to check out
[other parts](../../develop/develop-images/multistage-build.md) of the Docker
documentation. This is, however, not essential for our progress here, so we'll
leave it at that.

## Next steps

In this module, we took a look at setting up our example Go application that we
will use for much of the rest of the tutorial. We also created a `Dockerfile`
that we used to build our Docker image. Then, we took a look at tagging our
images and removing images and tags. In the next module, we’ll take a look at
how to:

[Run your image as a container](run-containers.md){: .button .outline-btn}

## Feedback

Help us improve this topic by providing your feedback. Let us know what you
think by creating an issue in the [Docker Docs](https://github.com/docker/docker.github.io/issues/new?title=[Golang%20docs%20feedback]){:target="_blank" rel="noopener" class="_"}
GitHub repository. Alternatively, [create a PR](https://github.com/docker/docker.github.io/pulls){:target="_blank" rel="noopener" class="_"}
to suggest updates.
