Feature request: Ability to configure build PATH

[zellio]

Context:
Currently dockerfile-mode uses default-directory to determine the execution PATH provided to the docker build command call. This doesn't play nicely with project setups that store their docker file in a subdirectory of the project, but need the project root as the docker build context.

Request:
Provide either an optional defcustom for build path or some function hook so users can utilize things like projectile or their own functions to locate the path they want to build from.