docs: link cwl-docker-extract from CWL_SINGULARITY_CACHE help (#1537)

[arimu1]

Summary

Addresses #1537, which asked to document CWL_SINGULARITY_CACHE and link to the cwl-utils Pull the all referenced software container images reference.

The variable is now listed in the environment-variable table (introduced when the CLI groups were reorganized), so it appears in cwltool --help and the autogenerated Sphinx CLI docs. What was still missing is the second half of the request: pointing users at how to pre-populate that cache.

Change

Extend the CWL_SINGULARITY_CACHE entry in cwltool/argparser.py to link to cwl-utils' cwl-docker-extract, which pre-pulls all referenced container images into a directory. It uses the existing [link=...] markup (same as the ORCID entry), so it renders correctly in both outputs:

cwltool --help (terminal hyperlink):

CWL_SINGULARITY_CACHE │ directory path to find already downloaded
                      │ Singularity images; dockerFile images will also be
                      │ searched for and stored here. Images can be
                      │ pre-pulled into this directory using
                      │ cwl-docker-extract from cwl-utils

Sphinx CLI docs (reStructuredText):

.. envvar:: CWL_SINGULARITY_CACHE

    directory path to find already downloaded Singularity images; ``dockerFile`` images
    will also be searched for and stored here. Images can be pre-pulled into this directory
    using `cwl-docker-extract <https://github.com/common-workflow-language/cwl-utils#pull-the-all-referenced-software-container-images>`__ from cwl-utils

Verified the cwl-utils anchor still resolves, the parser still builds, and black/flake8 are clean. Docs-only string change. Once this lands, #1537 can be closed.

---

This change was produced with the assistance of Claude Code (model: Claude Opus). It was reviewed by a human before submission.

[mr-c]

Probably better to link to https://pypi.org/project/cwl-utils/ or https://cwl-utils.readthedocs.io/en/latest/#cwl-docker-extract

[arimu1]

Good call — switched the link to the stable cwl-utils docs page: https://cwl-utils.readthedocs.io/en/latest/#cwl-docker-extract (confirmed the #cwl-docker-extract anchor is present). Pushed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.18%. Comparing base (5634342) to head (cb0e671).

Additional details and impacted files

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  Coverage   85.18%   85.18%           
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  Files          46       46           
  Lines        8593     8593           
  Branches     2011     2011           
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  Hits         7320     7320           
  Misses        807      807           
  Partials      466      466           

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[arimu1]

Updated per review — the link now points to https://cwl-utils.readthedocs.io/en/latest/#cwl-docker-extract instead of the GitHub README anchor. Also rebased onto current main.