Some backports from work on #46 to greatly speed up Circle builds

This backports from topic-docker the base image, which we can now use in
Circle to MASSIVELY speed up our CI situation.

The base image can currently only be pushed by @klardotsh as nobody else
has access to the Docker Hub org, but that can change in the future.

.gitignore

@@ -108,6 +108,7 @@ venv.bak/
 .circuitpy-deps
 .micropython-deps
 .devdeps
+.docker_base
 
 # Pycharms cruft
 .idea