fix(ci): do not build docker image if exists

[moliholy]

This PR skips docker build if it already exists in the repository.

[moliholy]

@AlexD10S I confirm this works. The docker image building is skipped, as it already exists.

[AlexD10S]

LGTM, thanks for the links shared!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.16%. Comparing base (8dd893c) to head (dda0f62).
⚠️ Report is 2 commits behind head on main.

@@           Coverage Diff           @@
##             main     #638   +/-   ##
=======================================
  Coverage   78.16%   78.16%           
=======================================
  Files         109      109           
  Lines       25464    25464           
  Branches    25464    25464           
=======================================
  Hits        19903    19903           
  Misses       3481     3481           
  Partials     2080     2080           

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