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How to use Docker and gemstash to not download Gems over and over again

Using Docker for Rails development is an obvious fit now. But it doesn’t come with only advantages. One of the most valid counter argument is the fact that you’ll download gems over and over again from Internet.

Even if your Dockerfile is written with these Best Practices in mind, you end up downloading every gem each time one is added to the Gemfile.

This post explains how to use gemstash running in a sidecar container to ease that pain.

Run gemstash in a container

Gemstash helps you run a cache or private gem server. We are going to use it as a proxy to

Here’s the two commands you’ll need to run gemstash in a container:

cat <<EOF | docker build -t gemstash -
FROM ruby:2.4.1

RUN gem install --no-ri --no-rdoc gemstash

VOLUME /root/.gemstash

ENTRYPOINT ["gemstash"]
CMD ["start", "--no-daemonize"]

docker run -d --name gemstash -v $(PWD)/gemstash:/root/.gemstash -p 9292:9292 gemstash

Change your app’s Dockerfile to use gemstash

Here’s the line that you should add to your existing Dockerfile to use gemstash as a proxy to

FROM ...

RUN bundle config mirror. http://docker.for.mac.localhost:9292

RUN bundle install

We tell bundle to connect to docker.for.mac.localhost for it now runs a rubygems proxy. Now, docker build will point at gemstash and all subsequent builds will be much faster.

FYI docker.for.mac.localhost is the magic hostname that makes your containers connect to your Mac. On Windows, replace docker.for.mac.localhost with

Or use docker build –network

Another (maybe cleaner) way of achieving the same goal without using docker.for.mac.localhost special hostname is to run the gemstash container on a specific docker network and run docker build with a --network flag:

docker network create rails
docker run -d --name gemstash -v $(PWD)/gemstash:/root/.gemstash --network rails gemstash

The modification to your Dockerfile will then be:

RUN bundle config mirror. http://gemstash:9292

This solution is more portable that using docker.for.mac.localhost special hostname. Also, it makes the gemstash container and the build container communicate directly without going through Docker for Mac’s vpnkit-based proxy.

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