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touch-up Markdown formatting and highlighting 

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
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Sebastiaan van Stijn 2019-03-08 17:37:23 +01:00
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develop/develop-images/dockerfile_best-practices.md
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@ -22,8 +22,8 @@ A Docker image consists of read-only layers each of which represents a
Dockerfile instruction. The layers are stacked and each one is a delta of the Dockerfile instruction. The layers are stacked and each one is a delta of the
changes from the previous layer. Consider this `Dockerfile`: changes from the previous layer. Consider this `Dockerfile`:
```conf ```Dockerfile
FROM ubuntu:15.04 FROM ubuntu:18.04
COPY . /app COPY . /app
RUN make /app RUN make /app
CMD python /app/app.py CMD python /app/app.py
@ -31,7 +31,7 @@ CMD python /app/app.py
Each instruction creates one layer: Each instruction creates one layer:
- `FROM` creates a layer from the `ubuntu:15.04` Docker image. - `FROM` creates a layer from the `ubuntu:18.04` Docker image.
- `COPY` adds files from your Docker client's current directory. - `COPY` adds files from your Docker client's current directory.
- `RUN` builds your application with `make`. - `RUN` builds your application with `make`.
- `CMD` specifies what command to run within the container. - `CMD` specifies what command to run within the container.
@ -270,8 +270,8 @@ frequently changed:
A Dockerfile for a Go application could look like: A Dockerfile for a Go application could look like:
``` ```Dockerfile
FROM golang:1.9.2-alpine3.6 AS build FROM golang:1.11-alpine AS build
# Install tools required for project # Install tools required for project
# Run `docker build --no-cache .` to update dependencies # Run `docker build --no-cache .` to update dependencies
@ -346,12 +346,14 @@ review. Adding a space before a backslash (`\`) helps as well.
Here’s an example from the [`buildpack-deps` image](https://github.com/docker-library/buildpack-deps): Here’s an example from the [`buildpack-deps` image](https://github.com/docker-library/buildpack-deps):
RUN apt-get update && apt-get install -y \ ```Dockerfile
bzr \ RUN apt-get update && apt-get install -y \
cvs \ bzr \
git \ cvs \
mercurial \ git \
subversion mercurial \
subversion
```
### Leverage build cache ### Leverage build cache
@ -416,7 +418,7 @@ The following examples show the different acceptable formats. Explanatory commen
> Strings with spaces must be quoted **or** the spaces must be escaped. Inner > Strings with spaces must be quoted **or** the spaces must be escaped. Inner
> quote characters (`"`), must also be escaped. > quote characters (`"`), must also be escaped.
```conf ```Dockerfile
# Set one or more individual labels # Set one or more individual labels
LABEL com.example.version="0.0.1-beta" LABEL com.example.version="0.0.1-beta"
LABEL vendor1="ACME Incorporated" LABEL vendor1="ACME Incorporated"
@ -430,14 +432,14 @@ to combine all labels into a single `LABEL` instruction, to prevent extra layers
from being created. This is no longer necessary, but combining labels is still from being created. This is no longer necessary, but combining labels is still
supported. supported.
```conf ```Dockerfile
# Set multiple labels on one line # Set multiple labels on one line
LABEL com.example.version="0.0.1-beta" com.example.release-date="2015-02-12" LABEL com.example.version="0.0.1-beta" com.example.release-date="2015-02-12"
``` ```
The above can also be written as: The above can also be written as:
```conf ```Dockerfile
# Set multiple labels at once, using line-continuation characters to break long lines # Set multiple labels at once, using line-continuation characters to break long lines
LABEL vendor=ACME\ Incorporated \ LABEL vendor=ACME\ Incorporated \
com.example.is-beta= \ com.example.is-beta= \
@ -476,26 +478,31 @@ know there is a particular package, `foo`, that needs to be updated, use
Always combine `RUN apt-get update` with `apt-get install` in the same `RUN` Always combine `RUN apt-get update` with `apt-get install` in the same `RUN`
statement. For example: statement. For example:
RUN apt-get update && apt-get install -y \ ```Dockerfile
package-bar \ RUN apt-get update && apt-get install -y \
package-baz \ package-bar \
package-foo package-baz \
package-foo
```
Using `apt-get update` alone in a `RUN` statement causes caching issues and Using `apt-get update` alone in a `RUN` statement causes caching issues and
subsequent `apt-get install` instructions fail. For example, say you have a subsequent `apt-get install` instructions fail. For example, say you have a
Dockerfile: Dockerfile:
FROM ubuntu:14.04 ```Dockerfile
RUN apt-get update FROM ubuntu:18.04
RUN apt-get install -y curl RUN apt-get update
RUN apt-get install -y curl
```
After building the image, all layers are in the Docker cache. Suppose you later After building the image, all layers are in the Docker cache. Suppose you later
modify `apt-get install` by adding extra package: modify `apt-get install` by adding extra package:
FROM ubuntu:14.04 ```Dockerfile
RUN apt-get update FROM ubuntu:18.04
RUN apt-get install -y curl nginx RUN apt-get update
RUN apt-get install -y curl nginx
```
Docker sees the initial and modified instructions as identical and reuses the Docker sees the initial and modified instructions as identical and reuses the
cache from previous steps. As a result the `apt-get update` is _not_ executed cache from previous steps. As a result the `apt-get update` is _not_ executed
@ -509,10 +516,12 @@ intervention. This technique is known as "cache busting". You can also achieve
cache-busting by specifying a package version. This is known as version pinning, cache-busting by specifying a package version. This is known as version pinning,
for example: for example:
RUN apt-get update && apt-get install -y \ ```Dockerfile
package-bar \ RUN apt-get update && apt-get install -y \
package-baz \ package-bar \
package-foo=1.3.* package-baz \
package-foo=1.3.*
```
Version pinning forces the build to retrieve a particular version regardless of Version pinning forces the build to retrieve a particular version regardless of
what’s in the cache. This technique can also reduce failures due to unanticipated changes what’s in the cache. This technique can also reduce failures due to unanticipated changes
@ -521,20 +530,22 @@ in required packages.
Below is a well-formed `RUN` instruction that demonstrates all the `apt-get` Below is a well-formed `RUN` instruction that demonstrates all the `apt-get`
recommendations. recommendations.
RUN apt-get update && apt-get install -y \ ```Dockerfile
aufs-tools \ RUN apt-get update && apt-get install -y \
automake \ aufs-tools \
build-essential \ automake \
curl \ build-essential \
dpkg-sig \ curl \
libcap-dev \ dpkg-sig \
libsqlite3-dev \ libcap-dev \
mercurial \ libsqlite3-dev \
reprepro \ mercurial \
ruby1.9.1 \ reprepro \
ruby1.9.1-dev \ ruby1.9.1 \
s3cmd=1.1.* \ ruby1.9.1-dev \
&& rm -rf /var/lib/apt/lists/* s3cmd=1.1.* \
&& rm -rf /var/lib/apt/lists/*
```
The `s3cmd` argument specifies a version `1.1.*`. If the image previously The `s3cmd` argument specifies a version `1.1.*`. If the image previously
used an older version, specifying the new one causes a cache bust of `apt-get used an older version, specifying the new one causes a cache bust of `apt-get
@ -630,10 +641,12 @@ variables specific to services you wish to containerize, such as Postgres’s
Lastly, `ENV` can also be used to set commonly used version numbers so that Lastly, `ENV` can also be used to set commonly used version numbers so that
version bumps are easier to maintain, as seen in the following example: version bumps are easier to maintain, as seen in the following example:
ENV PG_MAJOR 9.3 ```Dockerfile
ENV PG_VERSION 9.3.4 ENV PG_MAJOR 9.3
RUN curl -SL http://example.com/postgres-$PG_VERSION.tar.xz | tar -xJC /usr/src/postgress && … ENV PG_VERSION 9.3.4
ENV PATH /usr/local/postgres-$PG_MAJOR/bin:$PATH RUN curl -SL http://example.com/postgres-$PG_VERSION.tar.xz | tar -xJC /usr/src/postgress && …
ENV PATH /usr/local/postgres-$PG_MAJOR/bin:$PATH
```
Similar to having constant variables in a program (as opposed to hard-coding Similar to having constant variables in a program (as opposed to hard-coding
values), this approach lets you change a single `ENV` instruction to values), this approach lets you change a single `ENV` instruction to
@ -699,9 +712,11 @@ the specifically required files change.
For example: For example:
COPY requirements.txt /tmp/ ```Dockerfile
RUN pip install --requirement /tmp/requirements.txt COPY requirements.txt /tmp/
COPY . /tmp/ RUN pip install --requirement /tmp/requirements.txt
COPY . /tmp/
```
Results in fewer cache invalidations for the `RUN` step, than if you put the Results in fewer cache invalidations for the `RUN` step, than if you put the
`COPY . /tmp/` before it. `COPY . /tmp/` before it.
@ -712,16 +727,20 @@ delete the files you no longer need after they've been extracted and you don't
have to add another layer in your image. For example, you should avoid doing have to add another layer in your image. For example, you should avoid doing
things like: things like:
ADD http://example.com/big.tar.xz /usr/src/things/ ```Dockerfile
RUN tar -xJf /usr/src/things/big.tar.xz -C /usr/src/things ADD http://example.com/big.tar.xz /usr/src/things/
RUN make -C /usr/src/things all RUN tar -xJf /usr/src/things/big.tar.xz -C /usr/src/things
RUN make -C /usr/src/things all
```
And instead, do something like: And instead, do something like:
RUN mkdir -p /usr/src/things \ ```Dockerfile
&& curl -SL http://example.com/big.tar.xz \ RUN mkdir -p /usr/src/things \
| tar -xJC /usr/src/things \ && curl -SL http://example.com/big.tar.xz \
&& make -C /usr/src/things all | tar -xJC /usr/src/things \
&& make -C /usr/src/things all
```
For other items (files, directories) that do not require `ADD`’s tar For other items (files, directories) that do not require `ADD`’s tar
auto-extraction capability, you should always use `COPY`. auto-extraction capability, you should always use `COPY`.
@ -736,16 +755,22 @@ default flags).
Let's start with an example of an image for the command line tool `s3cmd`: Let's start with an example of an image for the command line tool `s3cmd`:
ENTRYPOINT ["s3cmd"] ```Dockerfile
CMD ["--help"] ENTRYPOINT ["s3cmd"]
CMD ["--help"]
```
Now the image can be run like this to show the command's help: Now the image can be run like this to show the command's help:
$ docker run s3cmd ```bash
$ docker run s3cmd
```
Or using the right parameters to execute a command: Or using the right parameters to execute a command:
$ docker run s3cmd ls s3://mybucket ```bash
$ docker run s3cmd ls s3://mybucket
```
This is useful because the image name can double as a reference to the binary as This is useful because the image name can double as a reference to the binary as
shown in the command above. shown in the command above.
@ -784,23 +809,31 @@ exec "$@"
The helper script is copied into the container and run via `ENTRYPOINT` on The helper script is copied into the container and run via `ENTRYPOINT` on
container start: container start:
COPY ./docker-entrypoint.sh / ```Dockerfile
ENTRYPOINT ["/docker-entrypoint.sh"] COPY ./docker-entrypoint.sh /
CMD ["postgres"] ENTRYPOINT ["/docker-entrypoint.sh"]
CMD ["postgres"]
```
This script allows the user to interact with Postgres in several ways. This script allows the user to interact with Postgres in several ways.
It can simply start Postgres: It can simply start Postgres:
$ docker run postgres ```bash
$ docker run postgres
```
Or, it can be used to run Postgres and pass parameters to the server: Or, it can be used to run Postgres and pass parameters to the server:
$ docker run postgres postgres --help ```bash
$ docker run postgres postgres --help
```
Lastly, it could also be used to start a totally different tool, such as Bash: Lastly, it could also be used to start a totally different tool, such as Bash:
$ docker run --rm -it postgres bash ```bash
$ docker run --rm -it postgres bash
```
### VOLUME ### VOLUME