doc: document ghcr as alternative to docker hub

[mccutchen]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.50%. Comparing base (ae8a6ed) to head (80dae3a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #202   +/-   ##
=======================================
  Coverage   94.50%   94.50%           
=======================================
  Files          10       10           
  Lines        2185     2185           
=======================================
  Hits         2065     2065           
  Misses         84       84           
  Partials       36       36           

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