Allow configuring path to dockerfiles

[aeons]

I'm not completely sure this is the way to do it, but it works :)

[teamon]

Hey,

I thought about this kind of quick solution too, but I think it will break mix docker.customize command (although all tests are passing). I'll try to take a look at this in few hours.

[aeons]

It works for me:

λ mix docker.customize
18:20:00.602 [warn]  rel/Dockerfile.build already exists
18:20:00.602 [warn]  rel/Dockerfile.release already exists

[teamon]

Thanks! :)