Docker ADD vs COPY instructions

While reviewing a change in Cookiecutter Django, a co-maintainer (Fábio C. Barrionuevo da Luz @luzfcb) pointed at a section of the Docker documentation outlining the subtle, but important, differences between ADD and COPY.

If you asked me yesterday, I wouldn’t have been able to tell you the difference, and this was indeed enlightening. My takeaways:

1. Start with COPY to copy files into the container, being from the host filesystem or from another stage.
2. If the file is only needed temporarily, use a RUN instruction with bind mount instead.
3. Use ADD if you need to put files into your container from a remote source.

I don’t expect to remember this in one month time, but hopefully this page will help me find the resource quickly next time I need it.