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A minimal example

The file Bockerfile.alpine is very minimal. It just creates new image from alpine:3.8 and set up an environment variable. Let's generate a Dockerfile from it:

$ ../bocker.sh Bockerfile.alpine > Dockerfile

A sample output is found under output/Dockerfile.alpine.

A non minimal example

To get started with this example, try the following command

$ mkdir enabled/ -pv
$ touch enabled/test.txt

# Check if there is any problem
$ ../bocker.sh -t Bockerfile.minimal

# Generate Dockerfile with `bocker.sh`.
$ ../bocker.sh Bockerfile.minimal > Dockerfile

# See if there is any difference from author's output.
$ diff output/Dockerfile.minimal Dockerfile

# Build your image
$ docker build -t bocker/test .

More examples

Some details for https://github.com/icy/docker/tree/master/bocker.

Base things

There are lot of common definitions that can be used everywhere. For example, maintainer information, base image, common functions for install Debian packages. This can be done by defining a base Bockerfile, and then reuse it later.

For example, a common mistake is to define a global environment variable DEBIAN_FRONTEND=noninteractive in the base image. But this may lead to confused error message when you enter your running container and try to install some package.

A safer way is to make that environment variable local. And a better way is to put them in a bash function, like this

ed_apt_install() {
  apt-get update
  DEBIAN_FRONTEND=noninteractive \
    apt-get install -y --no-install-recommends $@
}

(See https://github.com/icy/docker/blob/master/bocker/Bockerfile.base#L25.)

Now whenever you want to install a package, try /bocker.sh ed_apt_install foo (from your non-interactive shell), and/or in your other functions.

Overloading

Overloading can be done very simple thanks to Bash. To overload a method in a base image, you only need to reuse or source the base Bockefile, and redefine any function you want.

For example, the main line version of nginx differs from the stable version in the version number. With a careful design, the whole mainline image can be done as below

ed_reuse "$(dirname ${BASH_SOURCE[0]:-.})/Bockerfile.nginx"

ed_nginx_env() {
  export NGINX_VERSION=1.9.1
  export NGINX_CHECKSUM=b021d26bcefd41c8b9f9f35f263edf005e0f41dd

  export NGINX_VERSION=1.9.2
  export NGINX_CHECKSUM=814855ab98d6b0900207a6e5307252b130af61a2
}

This is actually done at https://github.com/icy/docker/blob/master/bocker/Bockerfile.nginx_mainline.

Refine the base image

Most of my images are based on debian:wheezy. But phantomjs requires a newer version of Debian, otherwise you end up in some fonts rendering issue. We can do that simply

ed_from  "debian:jessie"
ed_reuse "$(dirname ${BASH_SOURCE[0]:-.})"/Bockerfile.supervisor

This idea is impossible in the traditional Dockerfile because the (very) base image is fixed. You can only use it, or not; you can't refine them.

See https://github.com/icy/docker/blob/master/bocker/Bockerfile.phantomjs.