Low reputation so I can't comment inline (all those years of lurking, sigh), but in case anyone else happens across this while searching for ways to install old ruby versions to docker, I found @grosser's answer very helpful - it worked where trying to install via RVM simply wouldn't, at least for me.
I would, however, recommend using the recommended approach for installing ruby-build - the following worked for me:
<prior steps>
RUN git clone https://github.com/rbenv/ruby-build.git && \
PREFIX=/usr/local ./ruby-build/install.sh && \
ruby-build -v 2.4.1 /usr/local && \
gem install bundler -v <VERSION HERE> --no-ri --no-rdoc && bundle install
<following steps>
Key point here is that this keeps you up to date with ruby-build instead of being hard-coded to the 2018-03-29 version as in a previous @grosser's comment.