test_docker: drop git config step, copy maven cache to pwuser home

CI failure showed two issues with the unified-flag setup:
1. The Java Dockerfile does not install git, so the safe.directory
   config that the playwright-browsers workflow performs cannot run.
   The Java suite never invokes git inside the container, so the
   step is unnecessary.
2. The Dockerfile bakes the local maven cache into /root/.m2 (the
   build runs as root). When the container is launched as pwuser,
   that cache is unreadable, so playwright-java SNAPSHOT artifacts
   would not resolve. Copy it into /home/pwuser/.m2 with chown.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: yury-s

.github/workflows/test_docker.yml

@@ -54,8 +54,11 @@ jobs:
         run: |
             docker cp . "$CONTAINER_ID":/home/pwuser/playwright
             docker exec --user root "$CONTAINER_ID" chown -R pwuser /home/pwuser/playwright
-            docker exec --user root --workdir /home/pwuser/playwright "$CONTAINER_ID" \
-              git config --global --add safe.directory /home/pwuser/playwright
+            # Maven cache populated during image build lives in /root/.m2; make it
+            # accessible to pwuser so the suite doesn't have to redownload everything
+            # and so the locally-installed playwright-java SNAPSHOT artifacts are visible.
+            docker exec --user root "$CONTAINER_ID" cp -R /root/.m2 /home/pwuser/.m2
+            docker exec --user root "$CONTAINER_ID" chown -R pwuser /home/pwuser/.m2
 
       - name: Run test in container
         run: |