feat: Improved Dockerfile / build / test

[tarkatronic]

This change does a few things for us:

• Switch to using an official Go Docker image for the build so we can specify what version to build with.
• Distribute using a distroless image, which is theoretically slimmer. Funny enough this actually increases the size from 18.9mb, to 29.1mb. But that's fine, it's still pretty small as far as Docker images go.
  • This now builds the final image FROM scratch, taking it down to only 11.8mb! 🙌🏻
• Now using a non-root user to execute the bin, for a small security win
• Add a (by default unused) "test" stage to the Dockerfile.
• Test the Docker image during every CI build using that new "test" stage
• Add MacOS 13 to the build matrix
• Set up a CODEOWNERS file so we get PR review requests automatically. This is unrelated, but just necessary.

[codecov]

Codecov Report

Merging #67 (2102abd) into main (9fd00e9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #67   +/-   ##
=======================================
  Coverage   51.80%   51.80%           
=======================================
  Files          13       13           
  Lines         500      500           
=======================================
  Hits          259      259           
  Misses        238      238           
  Partials        3        3           

Flag	Coverage Δ
unittests	51.80% <ø> (ø)

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