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

Issue #1537 asked to document CWL_SINGULARITY_CACHE and link to the
cwl-utils "pull all referenced software container images" reference.

The variable itself is now listed in the environment-variable table
(shown in `cwltool --help` and the Sphinx CLI docs), but it did not
point users at how to pre-populate the cache. Add a link to
cwl-utils' cwl-docker-extract, using the existing [link=...] markup
so it renders both as a terminal hyperlink and as a reStructuredText
link in the generated docs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: arimu1

cwltool/argparser.py

@@ -66,7 +66,9 @@ def arg_parser() -> argparse.ArgumentParser:
     env_table.add_env(
         "CWL_SINGULARITY_CACHE",
         f"directory path to find already downloaded Singularity images; "
-        f"[{bt}]dockerFile[/] images will also be searched for and stored here",
+        f"[{bt}]dockerFile[/] images will also be searched for and stored here. "
+        f"Images can be pre-pulled into this directory using "
+        f"[link=https://github.com/common-workflow-language/cwl-utils#pull-the-all-referenced-software-container-images]cwl-docker-extract[/] from cwl-utils",
     )
     env_table.add_env(
         "ORCID",