Should we be storing the Docker layer cache?

[ferdnyc]

It strikes me that, since Docker images are built in layers, changing the set of packages installed with npm (which is like the 4th major layer of 5, in the image build) shouldn't require rebuilding the first three layers, if they're already in the Docker cache.

But that'd require us to use actions/cache (or a Docker-specific version of same) to store and retrieve the cache of intermediate layers, so that each build doesn't have to start from scratch.

Might be worth doing, at least for situations like my idiocy today. (Where I updated the npm install... step three times in quick succession, necessitating SIX (three PR CI, three post-merge deploy) redundant builds of the first three image layers.)