Dockerfile for Ruby applications

— July 18, 2015

There are different ways you can deploy your Ruby applications in a Docker container. You can either choose one of the many existing Ruby images on the public docker registry, use it as your base and adapt it to your needs or… build your own Ruby base image from scratch based on your favorite OS and then build your application image on top of it.

In this post we are going through the whole process of building the Ruby image from scratch based on the official Ubuntu 14.04 container. We will also create an onbuild image.

Building the base image

This image will set base for the rest of the post as we will use the resulting image to create our set of Ruby images.

The image will contain all the things Ruby expects to compile and run properly on a Debian based OS. I’m not going to talk about the required packages, instead I’m going to focus on the separation of concerns and configuration of the set of images.

One thing to keep in mind is that the image we’ll build is only for Ruby and will not contain any database related packages. If there’s a need to install a gem with native extensions requiring “extra” packages those should go into that specific image unless all of your apps require it.

What should you expect from this image?

This image will not complain about any TTY warnings during installation because of the noninteractive flag we are using, all your applications will use en_US.UTF-8 as the default encoding and most of the installation process will be silent do to the -qq flag.

FROM ubuntu:14.04.2
MAINTAINER Alberto Grespan <https://twitter.com/albertogg>

# Ignore TTY warnings on install
ENV DEBIAN_FRONTEND noninteractive

# configure Locale to en_US.UTF-8
RUN locale-gen en_US.UTF-8 &&\
    dpkg-reconfigure locales

# export LANG with en_US.UTF-8
ENV LANG en_US.UTF-8

# Quiet the update and install output
RUN apt-get update -qq && \
    apt-get install -y -qq \
      git \
      curl \
      wget \
      build-essential \
      ca-certificates \
      libyaml-dev \
      libreadline-dev \
      libcurl4-openssl-dev \
      libffi-dev \
      libgdbm-dev \
      libssl-dev \
      libxml2-dev \
      libxslt1-dev \
      libtool \
      zlib1g-dev \
      netbase

RUN rm -rf /var/lib/apt/lists/* &&\
    truncate -s 0 /var/log/*log

It’s a pretty straightforward image nothing rare so far. You can also use --no-install-recommends to avoid installing extra packages and save some space.

For building this image run the following command:

docker build -t dependency-image:14.04.2 .

note: If you need any database gem you can add any of the following packages and installing the database itself if required:

libpq-dev for PostgreSQL

libmysqlclient-dev for MySQL

libsqlite3-dev for SQLite3

Building the Ruby image

The Ruby image will use our base dependency image (the one we just build) and we’ll install Ruby version 2.2.2 through ruby-build and we’ll also install bundler by default.

If you need another Ruby version just change it in the RUBY_VERSION environment variable.

FROM dependency-image:14.04.2
MAINTAINER Alberto Grespan <https://twitter.com/albertogg>

# Set the Ruby version of your preference
ENV RUBY_VERSION 2.2.2

RUN echo 'gem: --no-document' >> /usr/local/etc/gemrc &&\
    mkdir /src && cd /src && git clone https://github.com/sstephenson/ruby-build.git &&\
    cd /src/ruby-build && ./install.sh &&\
    cd / && rm -rf /src/ruby-build && ruby-build $RUBY_VERSION /usr/local

RUN gem update --system &&\
    gem install bundler

This image still straightforward. We avoid gems installing documentation by setting the --no-document. You could also install any other “default” gems here.

For building this image run the following command:

docker build -t ruby:2.2.2 .

Building the onbuild image

The onbuild image is very practical if you are repeating over an over again the same steps in each application Dockerfile.

What this image does is execute all commands that exist in this image prior any commands on your application Dockerfile. For most ruby applications these commands shown here are almost always the same.

Commands will do:

Set the app directory
Copy the Gemfile and the Gemfile.lock to cache them out
Running bundle install
Copy the rest of the app files and remove the .env file if available.

like so:

FROM ruby:2.2.2
MAINTAINER Alberto Grespan <https://twitter.com/albertogg>

WORKDIR /usr/src/app

ONBUILD COPY Gemfile      /usr/src/app/
ONBUILD COPY Gemfile.lock /usr/src/app/
ONBUILD RUN bundle install --without test development

ONBUILD COPY . /usr/src/app
ONBUILD RUN rm -f .env

One thing to know about onbuild images is that they always have a custom docker tag with -onbuild in it. For example if our Ruby 2.2.2 image is named ruby with a tag 2.2.2 the onbuild image should be tagged 2.2.2-onbuild.

For building this image run the following command:

docker build -t ruby:2.2.2-onbuild .

How would an application Dockerfile look like while using onbuild image?

It would look something like this:

FROM ruby:2.2.2-onbuild

EXPOSE 3000

CMD ["foreman", "start"]

note: The CMD can also be added to onbuild Dockerfile if it’s always the same.

Thanks for reading!

By @albertogg