pass the docker group to the container so it can run docker tests

The ATH image no longer SUIDs the docker executable but relies on
permissions of the docker.sock.
This was needed to allow test-containers based tests to run as this uses
the socket directly rather than relying on the docker cli.

cf: jenkinsci/acceptance-test-harness#1706

ath.sh

@@ -26,11 +26,15 @@ fi
 mkdir -p target/ath-reports
 chmod a+rwx target/ath-reports
 
+# obtain the groupId to grant to access the docker socket to run tests needing docker
+dockergid=$(docker run --rm -v /var/run/docker.sock:/var/run/docker.sock ubuntu:noble stat -c %g /var/run/docker.sock)
+
 exec docker run --rm \
 	--env JDK \
 	--env ATH_VERSION \
 	--env BROWSER \
 	--shm-size 2g `# avoid selenium.WebDriverException exceptions like 'Failed to decode response from marionette' and webdriver closed` \
+	--group-add ${dockergid} \
 	--volume "$(pwd)"/war/target/jenkins.war:/jenkins.war:ro \
 	--volume /var/run/docker.sock:/var/run/docker.sock:rw \
 	--volume "$(pwd)"/target/ath-reports:/reports:rw \