Allow for Dockerfile to be named something else.

This is a follow-on to #7995 and #7284.

This PR allows for the Dockerfile to be named something other than `Dockerfile` via a -f/--file option. Aside from the name/path of the Dockerfile being variable, nothing else should change from today's behavior. The location of the Dockerfile MUST be within the build context.

This PR contains the:
- code changes
- docs updates
- testcases

Signed-off-by: Doug Davis <dug@us.ibm.com>
I found that last sentence hard to follow. Suggest something like: "Note that `debug` is in the directory structure of the build context, regardless of how you refer to it on the command line.