builtin/docker/builder: Add dockerfile variable to BuilderConfig

[xsaamiir]

Closes #538

Add the variable Dockerfile to the BuilderConfig of the Docker Plugin to support cases where the Dockerfile is not in the root of the project.

Please let me know if you think the variable should have another name or if you think the dockerfile name shouldn't be relative to the path of the app.

[mitchellh]

Looks great! I think in the future the only ask I'd have is to update the Docs function too. But I'll do that manually now.

[xsaamiir]

Looks great! I think in the future the only ask I'd have is to update the Docs function too. But I'll do that manually now.

Great thanks. Sure I will make sure to update the Docs function for any future contributions.