Port CircleCI improvements over from XCP-D

[tsalo]

Related to PennLINC/xcp_d#1596 and PennLINC/xcp_d#1597.

Changes proposed in this pull request

• Only build base image when the tag in Dockerfile is not already present on DockerHub.
• Only build production image when pixi.lock or Dockerfile are updated.
• Update maintenance documentation.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.24%. Comparing base (fe6e289) to head (253de0c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #628   +/-   ##
=======================================
  Coverage   76.24%   76.24%           
=======================================
  Files          40       40           
  Lines        5001     5001           
=======================================
  Hits         3813     3813           
  Misses       1188     1188           

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