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.

.dockerignore

@@ -0,0 +1,10 @@
+build
+vendor
+.devdeps
+.submodules
+.idea
+.micropython-deps
+.circuitpython-deps
+.ampy
+.venv
+venv