Allow for Dockerfile to be named something else

This adds a '-d' option to the "docker build" command to allow you to specify the name of the Dockerfile to use.
e.g.
docker build -d dev.dock -t myapp .
docker build -d prod.dock -t myapp .
This is really  handy when you have a docker context (dir tree) that is built multiple times/ways and the really only difference is the contents of the Dockerfile. 

While I think its functionally complete, I haven't updated the docs (just the cli help text) nor added testcases yet - as those will probably be a bigger job than the actual code changes :-) . I plan to do both but only if the idea of this PR is acceptable to folks.  If so, I'll then go back and finish the PR properly.

Comments?