Docker for Rails Developers

Caching Issue 1: Updating Packages

Currently, our Dockerfile has the following two lines:

RUN apt-get update -yqq
RUN apt-get install -yqq --no-install-recommends nodejs

Although this works, there’s a hidden problem lurking. Let’s say we come along at a later stage and realize we need to install an extra package—for example, the Vim editor. We add the vim package to the apt-get install RUN instruction, busting the cache and causing that instruction to be rerun:

RUN apt-get update -yqq
RUN apt-get install -yqq --no-install-recommends nodejs vim

However, the apt-get update RUN instruction remains unchanged, and the cached repository details will be used. Rather than getting the current, latest version of the new package we’ve added, ...