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>
the file must be within the build context, but is specified relaitve to where you're running the docker command - 

mmm, or maybe something to the effect of 'the file must begin with the path of the context'

imo confusing, but much more so if we don't document it (and the alternative is also confusing)