How to speed up your daily Docker builds

Nowadays, there is no way around Docker. It's a great technology that ensures that your software works on production the same as on your laptop.

If you have a medium size Node.JS or Ruby on Rails project, with a hand full of dependencies, this command can take a couple of minutes:

docker build -t orga/project:1.0.0 . 

Especially if you have dependencies with native extensions like libxml or sassc, the build will take a long time.

The build time can be reduced dramatically if you are using a Docker base image which includes already the majority of the needed dependencies. That's why I have, in most of my projects, a directory called docker_base, which contains a Dockerfile and package manager files for my base image. For a typical Ruby on Rails project the Dockerfile would look like this:

FROM ruby:2.5-alpine

ENV RAILS_ENV=production

WORKDIR /usr/src/app_base

COPY docker_base/Gemfile .
COPY package.json .

RUN apk update; \
    apk add build-base; \
    apk add libxml2-dev; \
    apk add libxslt-dev; \
    apk add ruby-nokogiri; \
    apk add yarn; \
    apk add tzdata; \
    yarn install --production=true; \
    bundle config build.nokogiri --use-system-libraries; \
    bundle install --without development test; 

The first COPY command adds a Gemfile with the most important dependencies. These Gemfile does not include private (closed source) dependencies. That way the Docker base image can be later hosted on a public Docker Hub repository. The second COPY command adds the package.json file for the frontend dependencies.

The RUN command is doing all the hard work which we want to avoid on our daily builds. It's installing all the native system libraries which are required for the Ruby and Node dependencies. In the last 3 lines we finally install all the Node and Ruby dependencies.

Let's build the base image like this:

docker build -t versioneye/base-web:1.0.0 . 

Now the regular Dockerfile in the root directory of the project can inherit from this base image. The first line of that Dockerfile would look like this:

FROM versioneye/base-web:1.0.0

The dependencies in your Gemfile and package.json might change every day. That's why it's important that you run the install steps again in your main Dockerfile! That way new dependencies will be installed. However, the build time shrinks dramatically because the base image contains already the majority of the dependencies. In my case I could reduce the build time from 5 minutes to under 1 minute!

What do you think about this method? Do you have another trick to reduce the build time of your daily Docker build?