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")

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #4965 from tianon/update-makefile

Update Makefile with several improvements