bundler installed gems not persisting in fig/docker - ruby-on-rails

Question

I am trying to use fig and docker to set up a local development environment for an existing rails app.
In the build process, I clearly see bundler installing the app's gems, but when I try to start the container with fig up or even reopen it with the /bin/bash command, the gems are not visible.
Here is the Dockerfile:
FROM ubuntu:14.04
# REPOS
RUN apt-get -qq update
RUN apt-get install -y software-properties-common
RUN add-apt-repository -y "deb http://archive.ubuntu.com/ubuntu $(lsb_release -sc) universe"
RUN add-apt-repository -y ppa:chris-lea/node.js
RUN apt-get -y update
#INSTALL
RUN apt-get install -y -q build-essential openssl libreadline6 libreadline6-dev curl git-core zlib1g zlib1g-dev libssl-dev libyaml-dev libsqlite3-dev sqlite3 libxml2-dev libxslt-dev autoconf libc6-dev ncurses-dev automake libtool bison pkg-config libpq-dev make wget unzip git vim nano nodejs gawk libgdbm-dev libffi-dev
#RUBY
RUN mkdir -p /download
WORKDIR download
RUN wget http://cache.ruby-lang.org/pub/ruby/2.1/ruby-2.1.2.tar.gz
RUN tar xvfz ruby-2.1.2.tar.gz
WORKDIR /download/ruby-2.1.2
RUN ./configure
RUN make
RUN make install
RUN gem update --system
RUN gem install bundler
RUN mkdir /rent
WORKDIR /rent
ADD Gemfile /rent/Gemfile
ADD Gemfile.lock /rent/Gemfile.lock
RUN bundle install --deployment
And here is the fig.yml file:
web:
build: .
command: bundle exec rails s -p 3000
volumes:
- .:/rent
ports:
- "3000:3000"
Running fig build clearly shows the app's gems being installed.
Running fig up fails with the message
bundler: command not found: rails
If I run fig run web /bin/bash and check the contents of
/local/lib/ruby/gems/2.1.0/gems
it only has bundler, rdoc, rake and a few others installed.
If I navigate to the app's directory and run the bundle command, it will install the app's gems and I can see that they are installed in the directory above. I can even start the app with rails server.
Why aren't the bundled gems being persisted in the image(container?).
I ran the rails tutuorial from the fig website and didn't have this problem.
Thanks

Answer 1

It seems that the problem was caused by using the --deployment flag with bundle install.
This flag's main effect is to deploy the gems to the vendor/bundle/ directory instead of the normal gem location. I checked and the gems were there, so I'm not sure why ruby couldn't find them.
Anyways, removing --deployment fixed the problem.

Answer 2

If you're using fig and mounted volumes, I found a solution which allows updates to the mounted files in development (like Gemfile.lock when you do fig run web bundle install) while still keeping container caching behavior.
See this for the full thing: https://gist.github.com/fotinakis/04077671bec4edf77c08
It's slightly convoluted, but basically you always install bundler and the gems as the non-root application user:
# Add 'web' user which will run the application.
RUN adduser web --home /home/web --shell /bin/bash --disabled-password --gecos ""
# Add directory where all application code will live and own it by the web user.
RUN mkdir /app
RUN chown -R web:web /app
# Install gems separately here to take advantage of container caching of `bundle install`.
# Also, ensure that gems are installed as the web user and not system-wide so that we can run
# `fig web bundle install` and the web user will have permissions to update the shared Gemfile.lock.
ADD Gemfile /app/
ADD Gemfile.lock /app/
RUN chown -R web:web /app
USER web
ENV HOME /home/web
ENV PATH $PATH:/home/web/.gem/ruby/2.1.0/bin
ENV GEM_HOME /home/web/.gem/ruby/2.1.0
ENV GEM_PATH $GEM_HOME
RUN gem install --user-install bundler
WORKDIR /app/
RUN bundle install
USER root
# Add the whole application source to the image and own it all by web:web.
# Note: this is overwritten in development because fig mounts a shared volume at /app.
ADD . /app/
RUN chown -R web:web /app
Running this will now work and update your local Gemfile.lock:
$ fig run web bundle install