pin Dockerfile `FROM`s to digests

We point our Dockerfiles at docker hub tags, but those tags can move at any point, and we may hit unexpected bustage. Recently, the python:3.8 docker tag moved to an image with an incompatible osslsigncode, which busted signingscript.

Ben suggested we pin our Dockerfiles to digests and explicitly bump them. Renovatebot can bump these (though we may have to do so manually if we specify the digest in an env var). This way we can explicitly decide when to roll out a new base image, after we've tested it for bustage.