ci: Pin Python base image in Dockerfile

[thompson-tomo]

Fixes # (issue)

Changes

In the latest patched pr, i noticed python image was not pinned, hence this pins it to the latest stable.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.80%. Comparing base (7c1e77c) to head (e79c8ba).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4677      +/-   ##
==========================================
- Coverage   86.81%   86.80%   -0.00%     
==========================================
  Files         525      525              
  Lines       20735    20735              
==========================================
- Hits        17999    17997       -2     
- Misses       2736     2738       +2     

see 1 file with indirect coverage changes

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