How can I prevent a Dockerfile instruction from being cached?

A build-time argument can be specified to forcibly break the cache from that step onwards. For example, in your Dockerfile, put

ARG CACHE_DATE=not_a_date

and then give this argument a fresh value on every new build. The best, of course, is the timestamp.

docker build --build-arg CACHE_DATE=$(date +%Y-%m-%d:%H:%M:%S) ...

Make sure the value is a string without any spaces, otherwise docker client will falsely take it as multiple arguments.

See a detailed discussion on Issue 22832.