Fix Jenkins workspace permission issue

- Add jenkins user to Dockerfile.ci and run as non-root
- Remove '-u root:root' args from Jenkinsfile Docker agent
- Build files will now be owned by jenkins user, allowing proper cleanup

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Entire-Checkpoint: f545f3d8aa33
Signed-off-by: twi-devel <torsten.wierschin@life-maps.com>

Jenkinsfile

@@ -3,7 +3,6 @@ pipeline {
     docker {
       image 'lifemaps/ci-cpp:latest'
       reuseNode true
-      args '-u root:root'
     }
   }