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

Jenkinsfile

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

ci/Dockerfile.ci

@@ -2,6 +2,10 @@ FROM ubuntu:26.04
 
 ENV DEBIAN_FRONTEND=noninteractive
 
+# Create jenkins user with same UID as host jenkins (typically 1000)
+RUN groupadd -g 1000 jenkins && \
+    useradd -u 1000 -g 1000 -m -s /bin/bash jenkins
+
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
       python3 \
@@ -30,3 +34,6 @@ ENV QT_QPA_PLATFORM=offscreen
 RUN git config --global --add safe.directory "*"
 
 WORKDIR /workspace
+
+# Switch to non-root user
+USER jenkins