--- 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. * You understand basic [Docker concepts](../../get-started/overview.md). * You're familiar with the [Dockerfile format](../../build/building/packaging.md#dockerfile). * You have [enabled BuildKit](../../build/buildkit/index.md#getting-started) on your machine. ## 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 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](../../build/building/base-images.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](../../build/building/multi-stage.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]({{ site.repo }}/issues/new?title=[Golang%20docs%20feedback]){:target="_blank" rel="noopener" class="_"} GitHub repository. Alternatively, [create a PR]({{ site.repo }}/pulls){:target="_blank" rel="noopener" class="_"} to suggest updates.