Took me a while to figure out how to get this running without installing ruby-dev or tools to compile native extensions, keeping my container nice and small.

Updated: using builtin packages for bundler + io-console now since they are smaller then installing manually and the logic is simpler
(apk add –update ruby ruby-io-console ruby-bundler)
… if latest bundler is needed below Dockerfile might still be useful.

FROM alpine

RUN apk add --update ruby && rm -rf /var/cache/apk/*

ENV BUNDLER_VERSION 1.12.3
RUN gem install bundler -v $BUNDLER_VERSION --no-ri --no-rdoc

# bundler wants some library that needs core extensions ... but it won't compile
RUN mkdir /usr/lib/ruby/gems/2.2.0/gems/bundler-$BUNDLER_VERSION/lib/io
RUN touch /usr/lib/ruby/gems/2.2.0/gems/bundler-$BUNDLER_VERSION/lib/io/console.rb

# bundler does not want to install as root
RUN bundle config --global silence_root_warning 1

RUN mkdir /app
WORKDIR /app

ADD Gemfile .
ADD Gemfile.lock .
RUN bundle