Do not fail if XDG_RUNTIME_DIR is not set

[jonesbusy]

Description

Do not fail if XDG_RUNTIME_DIR is not set

Fix #250

Also added a bit more tests that should help supporting better podman config to select correct credentials oras-project/oras-go#918 and containers-registries.conf based Container Registry alias etc....

Testing done

mvn clean install and add/update tests

Submitter checklist

• I have read and understood the CONTRIBUTING guide
• I have run mvn license:update-file-header, mvn spotless:apply, pre-commit run -a, mvn clean install before opening the PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.33%. Comparing base (c847a25) to head (71acd5c).
Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #255      +/-   ##
============================================
+ Coverage     85.31%   85.33%   +0.01%     
- Complexity      479      480       +1     
============================================
  Files            34       34              
  Lines          1594     1596       +2     
  Branches        178      179       +1     
============================================
+ Hits           1360     1362       +2     
  Misses          156      156              
  Partials         78       78              

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