OCI: Improve building

[amotl]

About

Further improve the OCI (building).

• Trim total cold run time, by also using slim-bookworm for build
• Serve grafana-wtf program per symlink instead of adjusting $PATH

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.51%. Comparing base (864d0e9) to head (93fc4f8).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #157   +/-   ##
=======================================
  Coverage   87.51%   87.51%           
=======================================
  Files           9        9           
  Lines        1089     1089           
=======================================
  Hits          953      953           
  Misses        136      136           

Flag	Coverage Δ
unittests	87.51% <ø> (ø)

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