Extract docker requirement source to single file

[Hneuschmidt]

Adds a single file (cwl/docker-requirement.yaml) that defines the docker image name (including the tag) for all cwl documents. This tag could then (still TODO) also be used to automatically generate and upload a docker image to the registry.

Unfortunately I have not found a way to extract only the tag (without the image name) to a single VERSION file, as the $include and $import directives of CWL preprocessing do not support string concatenation to my knowledge.

The docker image name / tag must be provided as a string and cannot be an expression, so we need to keep image name and tag combined in the same string.

In any case, this serves my purpose of having a single point to declare a version, it is just not as clean as hoped, the image name and registry must be prepended in the same file. This could be solved with a Makefile or similar, but that would require another manual step to generate cwl/docker-requirement.yaml before pushing.