How to install Ruby on docker?

This makes ruby available for any future RUN command and not just bash:

FROM debian:stretch-slim
RUN \
  apt-get update && apt-get install -y --no-install-recommends --no-install-suggests curl bzip2 build-essential libssl-dev libreadline-dev zlib1g-dev && \
  rm -rf /var/lib/apt/lists/* && \
  curl -L https://github.com/sstephenson/ruby-build/archive/v20180329.tar.gz | tar -zxvf - -C /tmp/ && \
  cd /tmp/ruby-build-* && ./install.sh && cd / && \
  ruby-build -v 2.5.1 /usr/local && rm -rfv /tmp/ruby-build-* && \
  gem install bundler --no-rdoc --no-ri

If you're starting FROM a different base Docker instance, you can simply RUN commands that install Ruby from your base instance's package management system. For example, this GitHub Gist shows how to use apt-get to install Ruby on a Ubuntu instance:

# Pull base image.
FROM dockerfile/ubuntu

# Install Ruby.
RUN \
  apt-get update && \
  apt-get install -y ruby

And this Gist shows a Dockerfile that's configured to install RVM and Ruby on a Ubuntu instance:

FROM ubuntu

RUN apt-get update

# basics
RUN apt-get install -y openssl

# install RVM, Ruby, and Bundler
RUN \curl -L https://get.rvm.io | bash -s stable
RUN /bin/bash -l -c "rvm requirements"
RUN /bin/bash -l -c "rvm install 2.0"
RUN /bin/bash -l -c "gem install bundler --no-ri --no-rdoc"