Pin python docker images to SHA

[VKTB]

Description

This PR pins the Python docker images in the Dockerfiles to commit SHAs so that Dependabot can create PRs each time there is a change in the upstream base images that the Python image is set to use as new SHAs are generated on every upstream change. Unfortunately, Dependabot only updates the Python versions but not the OS versions and there is an open issue for this. This means that we will have to do the Alpine version updates manually for now. This PR also configures Dependabot to ignore Python major and minor versions so again we will have to do such Python upgrades manually when we are ready to do that.

Testing Instructions

• Review code
• Check GitHub Actions build
• If icatdb Generator Script Consistency Test CI job fails, is this because of a deliberate change made to the script to change generated data (which isn't actually a problem) or is here an underlying issue with the changes made?
• Review changes to test coverage
• Does this change mean a new patch, minor or major version should be made? If so, does one of the commit messages feature fix:, feat: or BREAKING CHANGE: so a release is automatically made via GitHub Actions upon merge?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.66%. Comparing base (a9f336c) to head (dc9ec15).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #478   +/-   ##
=======================================
  Coverage   96.66%   96.66%           
=======================================
  Files          39       39           
  Lines        3242     3242           
  Branches      317      317           
=======================================
  Hits         3134     3134           
  Misses         80       80           
  Partials       28       28           

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