title: gem command not found when install rbenv in debian
body: <p>I'm creating a Dockerfile to run truffleruby. I'm getting an error when trying to install bundler and foreman. The error is <code>/bin/sh: 1: gem: not found</code></p>
  <h3>Dockerfile</h3>
  <pre><code>FROM debian:buster-slim
  
  # Install packages for building ruby
  RUN apt update -y &amp;&amp; apt install -y git curl libssl-dev libpq-dev libreadline-dev zlib1g-dev \
    autoconf bison build-essential libyaml-dev \
    libreadline-dev libncurses5-dev libffi-dev libgdbm-dev
  RUN apt clean
  
  # Install rbenv and ruby-build
  RUN git clone https://github.com/sstephenson/rbenv.git /root/.rbenv
  RUN git clone https://github.com/sstephenson/ruby-build.git /root/.rbenv/plugins/ruby-build
  RUN /root/.rbenv/plugins/ruby-build/install.sh
  ENV PATH /root/.rbenv/bin:$PATH
  RUN echo 'eval &quot;$(rbenv init -)&quot;' &gt;&gt; /etc/profile.d/rbenv.sh # or /etc/profile
  RUN echo 'eval &quot;$(rbenv init -)&quot;' &gt;&gt; .bashrc
  RUN . ~/.bashrc
  
  RUN rbenv install truffleruby-20.3.0
  RUN rbenv global truffleruby-20.3.0
  RUN rbenv rehash
  
  ENV BUNDLER_VERSION=2.2.4 NODE_ENV=production RAILS_ENV=production RAILS_SERVE_STATIC_FILES=true RAILS_LOG_TO_STDOUT=true PORT=3000
  ENV CONFIGURE_OPTS --disable-install-doc
  
  RUN apt-get install -y curl -sL https://deb.nodesource.com/setup_14.x | bash - &amp;&amp; \
    apt-get update &amp;&amp; apt-get install -y nodejs &amp;&amp; \
    apt-get clean
  
  RUN rbenv versions
  RUN gem install bundler:2.2.4 foreman
  
  RUN mkdir /app
  WORKDIR /app
  COPY Gemfile Gemfile.lock ./
  RUN bundle config set --local deployment 'true'
  RUN bundle config set --local without 'development test'
  RUN bundle install
  COPY . .
  
  
  EXPOSE 3000
  CMD [&quot;foreman&quot;, &quot;start&quot;]
  </code></pre>
  <h3>tail of build</h3>
  <pre><code>Removing intermediate container 1a445fde7fc0
   ---&gt; 43c3d72b7eb6
  Step 17/27 : RUN rbenv versions
   ---&gt; Running in feb5bb9361cc
  * truffleruby-20.3.0 (set by /root/.rbenv/version)
  Removing intermediate container feb5bb9361cc
   ---&gt; c7d1a5826af5
  Step 18/27 : RUN gem install bundler:2.2.4 foreman
   ---&gt; Running in 998461afc89c
  /bin/sh: 1: gem: not found
  The command '/bin/sh -c gem install bundler:2.2.4 foreman' returned a non-zero code: 127
  </code></pre>