Installing ruby via rbenv inside Debian Docker image

rbenv.Dockerfile

#jnlp-slave is based on slave, which in turn is based on scm-stretch debian 9.8
FROM jenkins/jnlp-slave:latest-jdk11

USER jenkins

# ==== Everything above this line is just to set an example context. Meaning - not important ====

# Install Local ruby
RUN git clone https://github.com/rbenv/rbenv.git ~/.rbenv \
  &&  echo 'export PATH="$HOME/.rbenv/bin:$PATH"' >> ~/.bashrc \
  &&  echo 'eval "$(rbenv init -)"' >> ~/.bashrc

ENV HOME /home/jenkins
ENV PATH "$HOME/.rbenv/bin:$HOME/.rbenv/shims:$PATH"
ENV RUBY_VERSION 2.6.3

RUN mkdir -p "$(rbenv root)"/plugins \
    && git clone https://github.com/rbenv/ruby-build.git "$(rbenv root)"/plugins/ruby-build

RUN rbenv install $RUBY_VERSION

RUN rbenv global $RUBY_VERSION && rbenv versions && ruby -v

# RUN curl -fsSL https://github.com/rbenv/rbenv-installer/raw/master/bin/rbenv-doctor | bash # Uncomment this to get rbenv to validate your setup.