Update Makefile with several improvements

Especially but not limited to:
- `make BINDDIR= ...` - for when you don't want a bind mount at all
- `make DOCSPORT=9000 docs` - for when you want a not-8000 docs port
- when we can't determine a branch name, we don't try to `docker build -t docker: .` anymore - we just `docker build -t docker .` (thus allowing Docker to assume ":latest")