Docker for Rails Developers

The Image Build Cache

During development, we rebuild our image fairly regularly, either to install new gems (bundle install is one of the steps in our Dockerfile) or to update our dependencies such as Node.js.

Just like a fast test suite helps by reducing the feedback loop, it’s important to keep our image builds as fast as possible too. One way that Docker helps is by caching each step in our build, meaning that it only needs to rebuild from the first instruction in the Dockerfile where there is a change. A change could either be literally deleting or modifying the Dockerfile instruction, or it could be associated with filesystem changes, as we’ll see shortly.

Since there are no changes to our Dockerfile or files since our last build, it would ...