Docker base image for RVM (Ruby Version Manager).

1. examples/Dockerfile.multi
   Automated installation of multiple Ruby versions via one simple ARG line

   docker build -t msati/docker-rvm .
   docker build -t example:multi - < examples/Dockerfile.multi
   docker run -it example:multi bash -l

Let's see how the Ruby 2.4 Integer Unification impacts us.

We'll create a directory containing a simple Dockerfile which runs irb:

ARG RVM_RUBY_VERSIONS="2.3 2.4"
FROM msati/docker-rvm
USER ${RVM_USER}
ENV RUBY=2.3
CMD rvm ${RUBY} do irb

Great! Let's build it.

docker build -t rvm-irb .

Ok, it's built. Let's run it:

docker run -it rvm-irb

2.3.4 :001 > 1.class
 => Fixnum

Great, now how easy is it to do the same in Ruby 2.4?

docker run -it -e RUBY=2.4 rvm-irb

2.4.1 :001 > 1.class
 => Integer

Very easy -- and no image rebuild necessary!

Please note that you'll probably want to use bash -l (that is, a login shell) for any interactive consoles, since that's the easiest way to use the RVM cli:

docker run -it rvm-irb bash -l

rvm@629159cda7a8:/$ rvm list

rvm rubies

=* ruby-2.3.4 [ x86_64 ]
   ruby-2.4.1 [ x86_64 ]

# => - current
# =* - current && default
#  * - default

And that's about it.

What use cases motivate dockerizing RVM, rather than using one of the official Ruby Docker images?

1. To easily switch between multiple Ruby versions in Docker, without rebuilding entire image on each switch
2. To employ RVM best practices, such as per-project gemsets, in Docker
3. To use an official Ruby installation method in Docker

Imagine that your team discovered that a piece of your code causes a crash in one version of Ruby (let's say 3.0.0-preview2), but not in another (2.7.2).

You've reported the issue to Ruby, and they've asked you to also check if the crash occurs in the trunk (aka head) version of Ruby.

Based off of this image, it's as easy as editing an ARG line at the top of your Dockerfile:

ARG RVM_RUBY_VERSIONS="2.7.2 3.0.0-preview2 ruby-head"
FROM msati/docker-rvm

# Now carry on as before -- building your project -- the base image will contain
# a layer in which all of those versions were installed by RVM.

# Later in your Dockerfile, or just interactively in a shell, switch versions
RUN rvm use --default ruby-head

Note that this also lowers the barrier for others in the community to jump in and work from your reproducible test case across Ruby versions.