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Assignment 6 - Utilizing layers

In order to speed up our build process, we should take advantage of Docker's build cache. Right now we re-install all gems whenever we make a change to our source code and rebuild the image. That takes time and is annoying. We can easily work around this by copying the Gemfile and Gemfile.lock separately. Change your Dockerfile so that it looks more like this:

FROM jfahrer/ruby:2.6.3-alpine3.10-ser

RUN apk add --update --no-cache \
      bash \
      build-base \
      nodejs \
      sqlite-dev \
      tzdata \
      postgresql-dev

RUN gem install bundler:2.0.2

WORKDIR /usr/src/app

COPY Gemfile Gemfile.lock ./

RUN bundle install

COPY . .

EXPOSE 3000

CMD ["rails", "server", "-b", "0.0.0.0"]

The key change here is that we copy our Gemfile and Gemfile.lock separately and run bundle install /before/ we copy all of the source code. As long as the two haven't changed, we can use the intermediate image from the build cache.

Rebuild the image with the changes in place:

docker image build -t your_docker_id/rails_app:v1 .

To make sure it works, let's make a small change to our source code. Simply add or change some text in app/views/books/index.html.erb and then rebuild the image. You should see that the RUN bundle install instruction is not executed but retrieved from the cache.

Bonus

  • Try invalidating the build cache by adding an empty line to your Gemfile and then rebuild the image.

What changed

You can find our changes in the utilizing_layers branch. Compare it to the previous branch to see what changed.

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