garethr / dockerfilepp-puppet

A small experiment in Dockerfile pre-processing

A very much proof-of-concept showing how simple it is to build Dockerfile pre-processors.

In this case dockerfilepp-puppet is a trivial go application which takes a Dockerfile on stdin and simply replaces some pre-defined values. The idea would be to make Dockerfile declarative again, making multiple Dockerfiles doing the same thing easier to maintain.

The examples centre around Puppet, but this is for demonstration purposes only. You could imagine building your own library of DSL extensions in a similar way, or extending into a general purpose tool. For this purpose most of the work has been split out into a sepatate library at github.com/garethr/dockerfile.

Problem

Dockerfile is wonderfully simple when it comes to hello-world examples, but the line-orientated nature and evolving best practices mean than it's commonn for some quite crazy imperative bash juggling to make it's way into what is best suited to a declarative build description. See the best practice for installing debian packages if you don't believe me.

The same hoops are often jumped through in multiple Dockerfiles, so the complex implementation details are copied and pasted into many places, making maintenance more costly.

Usage

So, given the following Dockerfile. We note that it is:

• Concise and declarative
• Totally not going to work if you pass it to docker because of the PUPPET_* instructions.

FROM ubuntu:16.04
MAINTAINER Gareth Rushgrove "[e-mail]"

ENV PUPPET_AGENT_VERSION="1.5.0" \
    R10K_VERSION="2.2.2" \
    UBUNTU_CODENAME="xenial"

PUPPET_INSTALL
PUPPET_COPY_PUPPETFILE
PUPPET_COPY_MANIFESTS
PUPPET_RUN

EXPOSE 80

CMD ["nginx"]

So lets run that through dockerfilepp-puppet.

First, build a binary. For this you'll need a Go environment, along with the the mentioned github.com/garethr/dockerfilepp dependency. You can install that with go get, or using the glide package manager, simple type glide up.

The project uses go-bindata to make management of processors easier so first install that. This is only required for building your own binaries, not using the resulting tool. At some point I'll start releasing precompiled binaries.

Running make build should then be enough to generate a dockerfilepp-puppet binary.

make build

Once you have the binary you can use it like so:

cat Dockerfile | ./dockerfilepp-puppet

This should output to stdout with a new Dockerfile which is:

• Much more verbose and much more imperative
• Going to work as an input to docker build

FROM ubuntu:16.04
MAINTAINER Gareth Rushgrove "[e-mail]"

ENV PUPPET_AGENT_VERSION="1.5.0" \
    R10K_VERSION="2.2.2" \
    UBUNTU_CODENAME="xenial"

RUN apt-get update && \
    apt-get install -y wget=1.17.1-1ubuntu1 && \
    wget https://apt.puppetlabs.com/puppetlabs-release-pc1-"$UBUNTU_CODENAME".deb && \
    dpkg -i puppetlabs-release-pc1-"$UBUNTU_CODENAME".deb && \
    rm puppetlabs-release-pc1-"$UBUNTU_CODENAME".deb && \
    apt-get update && \
    apt-get install --no-install-recommends -y puppet-agent="$PUPPET_AGENT_VERSION"-1"$UBUNTU_CODENAME" && \
    apt-get clean && \
    rm -rf /var/lib/apt/lists/*

RUN /opt/puppetlabs/puppet/bin/gem install r10k:"$R10K_VERSION" --no-ri --no-rdoc

COPY Puppetfile /
COPY manifests /manifests

RUN apt-get update && \
    /opt/puppetlabs/puppet/bin/r10k puppetfile install --moduledir /etc/puppetlabs/code/modules && \
    /opt/puppetlabs/bin/puppet apply manifests/init.pp --verbose --show_diff  --summarize && \
    apt-get clean && \
    rm -rf /var/lib/apt/lists/*

EXPOSE 80

CMD ["nginx"]