Add option to build from definition directory

[JeroenKnoops]

Feature

When using a git submodule for creating images of complete repositories, we need to be able to run the docker build script from the directory with the submodule instead of the root.

Changed

This was the way it worked in v2.2.2 and has been changed in v3.0.0

Example

https://github.com/philips-software/docker-ort

Proposal

Add option: build-from-directory: true to get the old functionality back.

[JeroenKnoops]

Will add: base-dir: