Allow setting a different filename for the Dockerfile

[cap10morgan]

I would like to be able to tell Docker to use a file other than Dockerfile but still keep the directory its in as the build context.

I'm happy to do the implementation and submit a PR, but I wanted to make sure there wasn't a philosophical incompatibility first.

My use case is that I have been creating Dockerfile.build files which define my build environment for a few different projects, and then separate Dockerfiles that define the runtime environment. I don't want everything upstream in the build container in my runtime container, just the build artifacts.

But it's been pretty painful getting Docker to work with the Dockerfile.build files because I have to use docker build - which means I don't get a build context.

[SvenDowideit]

there see #2112 and peterbraden@801acaa

The diff will need updating and a test and documentation :)

conversely there is another PR for allowing you to have multiple TAG's in one Dockerfile, which I like quite a bit :) #3251

[cap10morgan]

@SvenDowideit Oh... I quite like #3251 too. :)

[crosbymichael]

Dup of #2112