* Slim down contracts-bedrock package

The contracts-bedrock package was over 2GB in size, mostly because it contained the Git repository. It also supported multiple platforms, which would lead to costly recompilation steps every time the image was used on a different platform (e.g., OSX).

The PR updates the Dockerfile to no longer ship the Git repo. It also updates the Debian base image to a more modern, slimmed down version. The new image is <1G in size.

* fix bakefile