docs: add ghcr.io pull command to Docker section

[baubuchon-cf]

The Docker image is published to ghcr.io, not Docker Hub. The README mentioned a Docker image exists but never provided the registry location, causing customer confusion. Adds the pull command and corrects the registry reference.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.35%. Comparing base (3835228) to head (15142fb).
⚠️ Report is 29 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #562      +/-   ##
==========================================
+ Coverage   45.85%   47.35%   +1.49%     
==========================================
  Files          28       28              
  Lines        3670     3003     -667     
==========================================
- Hits         1683     1422     -261     
+ Misses       1815     1410     -405     
+ Partials      172      171       -1     

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[baubuchon-cf]

cc @dt-dtran @lgarofalo — this adds the ghcr.io pull command to the Docker section of the README. The image location was never documented, causing customer confusion. Would appreciate your review to confirm the content is accurate before merging. Thanks!