dynamic `DockerRequirement.dockerPull`

[mr-c]

We have gotten this requirement often.

https://cwl.discourse.group/t/dynamic-changing-docker-pull-registry-location-in-dockerrequirements/530
https://cwl.discourse.group/t/using-javascript-in-requirements-such-as-dockerpull/814
(and many other times)

It would also improve the WDL to CWL converter, as WDL workflows often have one or more string inputs used to specify the name of a container for some particular step(s).

However some CWL workflow runners, like Arvados, want to know the names of all containers at workflow submission time (to copy them to shared storage)

@tetron suggests the addition of a { class: ContainerName, dockerPull: user_provided_value } or similar. Then a workflow runner can scan the inputs object for class: ContainerName (and check the whole workflow DAG for any static class: ContainerNames) and handle the copying of those containers ahead of time.

[tetron]

Thinking a little more about this, this might also be an opportunity to redefine/migrate containers in CWL to reference the OCI image format and OCI image distribution spec for fetching/using container environments.