What You Learned

Each instruction leaves a layer, and a layer is the unit of both caching and disk.
The cache is a chain: once a layer changes, everything below it runs again.
Copy the manifest, install, then copy the source, so an edit cannot invalidate the install.
docker system df says what is reclaimable, and the build cache is usually most of it.
prune --volumes is the dangerous one, because a stopped database's data looks unused.