Lessons Learned In Writing Dockerfiles

Writing dockerfiles is pretty straight forward. Nevertheless, a little extra care goes a long way. Docker’s own Best Practices are a great starting point, covering everything from formatting to use of certain commands. In Rocker, We’ve tried to follow all of these suggestions and have found them very helpful. In particular:

• Minimize the number of layers, but use \ to break commands across multiple lines,

• Always run apt-get update && apt-get install -y ... rather than running updates in one layer and install in a different layer,

• Use COPY instead of ADD, WORKDIR instead of cd,

• Create a non-root user

This was tricky since we intend (and use) our dockerfiles as base images for other Dockerfiles. Setting the default user with USER would interfere with this (adding that user to sudoers, having to use sudo apt-get update or switch back to USER root and back again. Both sudo and switching back and forth on USER are discouraged in best practices)