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set-square

set-square is a tool to streamline the building of Docker images, allowing the use of simple placeholders within Dockerfiles.

The main idea was borrowed from wking's approach, and later rewritten from scratch.

Motivation

This tool allows building Docker images from Dockerfile templates. It relies upon envsubst, so fear not: it doesn't convert Dockerfiles into Turing machines. It focuses exclusively on allow using variable placeholders which get resolved at build time.

If you build your own images for third-party services such us Tomcat, MariaDB, RabbitMQ, etc., and the only difference from the image's point of view is the version of the package it bundles, then set-square alleviates you from the hassle of constantly maintaining almost-identical dockerfiles.

set-square uses dry-wit, and out-of-the-box it supports default values for variables. The user can easily choose which variables to override, and which don't.

Installation

git clone --recurse-submodules https://github.com/rydnr/set-square
cd set-square
git submodules init
git submodules update

Example

PostgreSQL image

Let's say you want to define your own PostgreSQL image (for whatever reasons) based on the official Dockerfile (available in github).

# vim:set ft=dockerfile:
FROM debian:jessie

# explicitly set user/group IDs
RUN groupadd -r postgres --gid=999 && useradd -r -g postgres --uid=999 postgres

[..]

ENV PG_MAJOR 9.3
ENV PG_VERSION 9.3.10-1.pgdg80+1

RUN echo 'deb http://apt.postgresql.org/pub/repos/apt/ jessie-pgdg main' $PG_MAJOR \
    > /etc/apt/sources.list.d/pgdg.list

RUN apt-get update \
	&& apt-get install -y postgresql-common \
	&& sed -ri 's/#(create_main_cluster) .*$/\1 = false/' /etc/...
	&& apt-get install -y \
		postgresql-$PG_MAJOR=$PG_VERSION \
		postgresql-contrib-$PG_MAJOR=$PG_VERSION \
	&& rm -rf /var/lib/apt/lists/*

RUN mkdir -p /var/run/postgresql && chown -R postgres /var/run/postgresql

ENV PATH /usr/lib/postgresql/$PG_MAJOR/bin:$PATH
ENV PGDATA /var/lib/postgresql/data
VOLUME /var/lib/postgresql/data

COPY docker-entrypoint.sh /

ENTRYPOINT ["/docker-entrypoint.sh"]

EXPOSE 5432
CMD ["postgres"]

However, you'd notice you'd like to specify certain information only when building the image, to avoid modifying the Dockerfile every time it changes. For example:

  • PG_MAJOR and PG_MINOR,
  • The Debian version it's based on,
  • The uid and gid of the internal Postgres user account.

The Dockerfile you're thinking of would be the following:

# vim:set ft=dockerfile:
FROM debian:$DEBIAN_VERSION

# explicitly set user/group IDs
RUN groupadd -r postgres --gid=$POSTGRES_GID \
    && useradd -r -g postgres --uid=$POSTGRES_UID postgres

[..]
# ENV PG_MAJOR 9.3
# ENV PG_VERSION 9.3.10-1.pgdg80+1

RUN echo 'deb http://apt.postgresql.org/pub/repos/apt/ jessie-pgdg main' $PG_MAJOR \
    > /etc/apt/sources.list.d/pgdg.list

RUN apt-get update \
	&& apt-get install -y postgresql-common \
	&& sed -ri 's/#(create_main_cluster) .*$/\1 = false/' /etc/...
	&& apt-get install -y \
		postgresql-$PG_MAJOR=$PG_VERSION \
		postgresql-contrib-$PG_MAJOR=$PG_VERSION \
	&& rm -rf /var/lib/apt/lists/*

RUN mkdir -p /var/run/postgresql && chown -R postgres /var/run/postgresql

ENV PATH /usr/lib/postgresql/$PG_MAJOR/bin:$PATH
ENV PGDATA /var/lib/postgresql/data
VOLUME /var/lib/postgresql/data

COPY docker-entrypoint.sh /

ENTRYPOINT ["/docker-entrypoint.sh"]

EXPOSE 5432
CMD ["postgres"]

The differences are:

  • PG_MAJOR and PG_MINOR are now Dockerfile variables, not environment variables in the image itself. Of course, we could leave the original ENV steps if we wanted to, but they are useless(*) and generate unnecessary cache layers.
  • DEBIAN_VERSION, POSTGRES_UID and POSTGRES_GID are now placeholders.

Building the image

First, place this Dockerfile template as postgres/Dockerfile.template. Secondly, create a new settings file (postgres/build-settings.sh) for specifying the default value you will be using normally:

defineEnvVar DEBIAN_VERSION "The Debian version" "jessie";
defineEnvVar PG_MAJOR "The PostgreSQL major version" "9.3";
defineEnvVar PG_VERSION "The complete PostgreSQL version" '${PG_MAJOR}.10-1.pgdg80+1';
defineEnvVar POSTGRES_UID "The uid of the PostgreSQL system account" "999";
defineEnvVar POSTGRES_GID "The gid of the PostgreSQL system account" '${POSTGRES_UID}';

The syntax of this minimal DSL is provided by dry-wit:

defineEnvVar NAME DESCRIPTION DEFAULT_VALUE;

And can be overridden (in postgres/.build-settings.sh) with

overrideEnvVar NAME NEW_VALUE;

You can now run set-square:

./build.sh -vv postgres

set-square will just transform any file within that folder which ends in *.template, and then ask Docker to build the image.

You'll probably notice in the example above that some of the variables' values use double quotes, while and some others use single quotes. If the values contain references to other variables, they have to use single quotes. Otherwise the value gets resolved by Bash itself, and the referenced variables won't be available and would me replaced by empty strings.

Phusion-based images

You can review a number of Phusion-based images built Using set-square in https://github.com/rydnr/set-square-phusion-images.

Documentation

set-square is a dry-wit-based Bash script. It should be self-explanatory and easy to read (and customize or extend), once you learn the basics. For a brief introduction to dry-wit, you can review this 10-minute slides.

In the docs folder you can find some slides describing both the tool and the motivation behind the images implemented using set-square.

(*) Even if they're used also at runtime, it's just a matter to rename the files using them as .template. Such files will end up using the correct values in the resulting Docker image.