run as the configured sbt-native-packager user instead of root

[bpholt]

Fixes #3283.

This works for us—the container runs and is able to clone, make PRs, etc., when running as the default user (demiourgos728 / UID 1001).

For fresh installs, I don't think any changes are needed to how one would run the container. However, for existing installs where the containers has been writing to some persistent storage as root, there may be some permissions issues. I'm not sure how to handle that.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.19%. Comparing base (752757f) to head (b931cb6).
Report is 684 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3286   +/-   ##
=======================================
  Coverage   91.19%   91.19%           
=======================================
  Files         167      167           
  Lines        3408     3408           
  Branches      304      304           
=======================================
  Hits         3108     3108           
  Misses        300      300           

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