gem command not found when install rbenv in debian - ruby

Question

I'm creating a Dockerfile to run truffleruby. I'm getting an error when trying to install bundler and foreman. The error is /bin/sh: 1: gem: not found
Dockerfile
FROM debian:buster-slim
# Install packages for building ruby
RUN apt update -y && 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:$PATH
RUN echo 'eval "$(rbenv init -)"' >> /etc/profile.d/rbenv.sh # or /etc/profile
RUN echo 'eval "$(rbenv init -)"' >> .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 - && \
apt-get update && apt-get install -y nodejs && \
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 ["foreman", "start"]
tail of build
Removing intermediate container 1a445fde7fc0
---> 43c3d72b7eb6
Step 17/27 : RUN rbenv versions
---> Running in feb5bb9361cc
* truffleruby-20.3.0 (set by /root/.rbenv/version)
Removing intermediate container feb5bb9361cc
---> c7d1a5826af5
Step 18/27 : RUN gem install bundler:2.2.4 foreman
---> 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

Answer 1

You don't generally use version managers like rbenv in Docker. There are a couple of reasons for this. One is that an image usually only contains a single application and its single runtime, so you'd never have more than one Ruby in an image and therefore there's no need to switch. A second is that most common paths of running containers (including docker run and the Dockerfile RUN directive) don't look at shell dotfiles like .bashrc or /etc/profile, so the version manager setup will never get run.
TruffleRuby is distributed (among other ways) as a standalone tar file so you can just install that in your Dockerfile. I'd make the Dockerfile look roughly like:
FROM debian:buster-slim
# Install the specific dependency packages TruffleRuby recommends
# (build-essential is much larger but might actually be necessary)
RUN apt-get update \
&& DEBIAN_FRONTEND=noninteractive \
apt-get install --no-install-recommends --assume-yes \
curl \
gcc \
libssl-dev \
libz-dev \
make
# Download and unpack TruffleRuby
ARG TRUFFLERUBY_VERSION=20.3.0
ENV PATH /opt/truffleruby-$TRUFFLERUBY_VERSION-linux-amd64/bin:$PATH
RUN cd /opt \
&& curl -L https://github.com/oracle/truffleruby/releases/download/vm-$TRUFFLERUBY_VERSION/truffleruby-$TRUFFLERUBY_VERSION-linux-amd64.tar.gz | tar xz \
&& /opt/truffleruby-$TRUFFLERUBY_VERSION-linux-amd64/lib/truffle/post_install_hook.sh
# Now build and install your application
RUN gem install bundler:2.2.4
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 . .
ENTRYPOINT ["bundle", "exec"]
EXPOSE 3000
CMD ["rails", "start"]
You can reasonably split this into two separate Dockerfiles. End the first one before the "build and install your application" comment, and build it with docker build -t myname/truffleruby:20.3.0 -f Dockerfile.truffleruby .. Then the second one can begin with FROM myname/truffleruby:20.3.0 in the same way as the standard Docker Hub ruby image.

Answer 2

Does the same work with CRuby?
I'd suspect RUN doesn't read shell files, so PATH needs to be modified.
RUN git clone https://github.com/sstephenson/rbenv.git /root/.rbenv
...
RUN /root/.rbenv/plugins/ruby-build/install.sh
ENV PATH /root/.rbenv/bin:$PATH
seems a bit weird, is rbenv cloned and installed in the same place?
I would skip rbenv in Docker, and instead just:
RUN ruby-build truffleruby ~/.rubies/truffleruby
ENV PATH $HOME/.rubies/truffleruby/bin:$PATH