Update Alpine base image to v3.18 in Dockerfile

[PeterDaveHello]

Alpine Linux v3.12 & v3.13 has reached its End of Life (EOL) already many months ago

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.74%. Comparing base (d8f628b) to head (a5186fa).
⚠️ Report is 9 commits behind head on master.
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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1798   +/-   ##
=======================================
  Coverage   46.74%   46.74%           
=======================================
  Files         690      690           
  Lines       45701    45701           
=======================================
  Hits        21365    21365           
  Misses      24336    24336           

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