fix: Make just container runner configurable in justfile

[oliwenmandiamond]

On machines that use podman instead of docker and don't have an alias in place, just needs to use podman. To support both, the runner still uses docker by default but can be configured to use podman via

$ just RUNNER=podman services

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.96%. Comparing base (5d7e2fa) to head (13a2ba7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1634   +/-   ##
=======================================
  Coverage   95.96%   95.96%           
=======================================
  Files          45       45           
  Lines        3323     3323           
=======================================
  Hits         3189     3189           
  Misses        134      134           

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