Building Docker images using Puppet

NB: The below is now out of date, as the stuff in this repository makes use of Puppet's official container image build module - see https://github.com/puppetlabs/puppetlabs-image_build. This module largely abstracts away the complexity below and makes things a fair bit more convenient.

This repository tracks files necessary to build my personal Docker images using Puppet to manage configuration. There's a corresponding blog post that goes into a bit more detail here: http://dischord.org/2016/09/10/more-on-docker-and-puppet/

Overview

Images are built using Puppet via puppet apply; Puppet applies once during the build process and configures all functionality required to run a particular application in a container.

This process makes use of a tool called 'Rocker' which provides additional options during the build process such as the ability to mount volumes that persist across build as well as templating capability. Rocker is a pre-requisite for building any of these images.

All images are generated from the same base configuration and Rockerfile template that exists in common.

Puppet code resides under puppet, and includes Hiera (configuraton) data.

Per-image Puppetfiles under puppet/r10k. This approach facilitates module version independance across image build, but duplication is kept to a minimum thanks to Rocker's shared volumes.

Building an image

Building an existing image is a case of running rocker build with a few options. From the repository directory:

$ rocker build -f common/Rockerfile --vars common/common.yaml \
  --var EXPOSE="80" --var TAG=dischord:webserver --var ROLE=webserver .

NB Don't miss off the trailing . as this is used to determine the relative path to various files!

This command ensures Puppet inherits the webserver role which declares various classes and scopes some parameters in Hiera. If all's well, you'll ned up with a Docker image tagged as dischord:webserver.

The can also override any of the variables templated in the Rockerfile. So for example, to build an image based off Debian 'jessie' instead of Ubuntu 'xenial':

$ rocker build -f common/Rockerfile --vars common/common.yaml \
--var EXPOSE="3306" --var TAG=dischord:database --var ROLE=database \
--var BASE="debian:jessie" --var DISTRO_CODENAME="jessie" .

No guarantees that swapping out the base image will always work, however - it depends on how well behaved some of the Puppet code is....