Get container before babs submit if missing

[tien-tong]

Closes #375. Make babs submit ensure that te configured container images are available before scheduler submission.

Changes:

• Added submit-time container image check/fetch.
• Skips datalad get when the image already exists.
• Fails before job submission if a missing image cannot be retrieved.
• Stores container_images in new project config, with fallback support for older configs.
• Removed obsolete manual container datalad get steps from docs, e2e script, and tests.
• Added tests for fetch, skip, and failure behavior.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.49%. Comparing base (95fd99d) to head (97983e1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #380      +/-   ##
==========================================
+ Coverage   79.07%   79.49%   +0.41%     
==========================================
  Files          17       17              
  Lines        2079     2121      +42     
  Branches      362      373      +11     
==========================================
+ Hits         1644     1686      +42     
  Misses        300      300              
  Partials      135      135              

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[mattcieslak]

Looks great! Nice tests and simplifies the e2e workflow