chore(docker): Extract .NET version to a variable

[fviernau]

See individual commits.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (448a8bb) 67.06% compared to head (adb0b3d) 67.06%.

❗ Current head adb0b3d differs from pull request most recent head 5e0d7f3. Consider uploading reports for the commit 5e0d7f3 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #7865   +/-   ##
=========================================
  Coverage     67.06%   67.06%           
  Complexity     2041     2041           
=========================================
  Files           356      356           
  Lines         17045    17045           
  Branches       2438     2438           
=========================================
  Hits          11431    11431           
  Misses         4594     4594           
  Partials       1020     1020           

Flag	Coverage Δ
funTest-non-docker	34.95% <ø> (+0.44%)	⬆️
test	36.10% <ø> (ø)

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[fviernau]

I've tried upgrading to 8.0, but it seems that in order to analyze our test projects, .NET version 6.0 has to be installed.
E.g. with only 8.0, it fails with No instances of MSBuild could be detected.. So, I've dropped the upgrade commit.