Wishlist: Add a feature to support installing packages in the Docker image when cross runs

I know I can use custom Docker images for this but creating and maintaining several (possibly 10+) images just to add a git binary seems very heavyweight and painful.

I would think that it'd be possible for cross to invoke apt-get in the image before it calls cargo (or xargo). I could look into implementing this if there's agreement that this is a thing that should be possible.