Customizing also Dockerfile paths in scrapinghub.yml

[Peng-Yu Chen]

Currently the shub image build or similar commands accepts an optional argument -f/--file for specifying Dockerfile files in custom paths (thanks to #302).

This is a proposal to further enhance this feature by allowing such customization in the scrapinghub.yml configuration file. Example:

projects:
  foo:
    id: 1
    image: true
    stack: null
    dockerfile: ./Dockerfile-foo
  bar:
    id: 2
    image: true
    stack: null
    dockerfile: ./Dockerfile-bar