ci: push to ghcr.io in addition to docker hub

[mccutchen]

Motivated by Docker's much more aggressive rate limits on unauthenticated pulls of public images going into effect on April 1:

Unauthenticated users and users with a free Personal account have the following pull limits:

• Unauthenticated users: 10 pulls/hour

Just gonna merge this and stamp a new release to see what happens, I guess?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.50%. Comparing base (05a9dfd) to head (ce3c6d4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #200   +/-   ##
=======================================
  Coverage   94.50%   94.50%           
=======================================
  Files          10       10           
  Lines        2185     2185           
=======================================
  Hits         2065     2065           
  Misses         84       84           
  Partials       36       36           

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