[Jenkins] Get gid of file instead of group

/var/run/docker.sock may not be owned by the docker group in this docker
image due to the gid on the host being different. So instead, get the
gid by using stat on the file and add those permissions in the inside
image.

Jenkinsfile

@@ -129,7 +129,7 @@ pipeline {
                 script {
                   def deps = docker.build('testsuite-fmu-crosscompile', '--pull .CI/cache')
                   // deps.pull() // Already built...
-                  def dockergid = sh (script: 'getent group docker | cut -d: -f3', returnStdout: true).trim()
+                  def dockergid = sh (script: 'stat -c %g /var/run/docker.sock', returnStdout: true).trim()
                   deps.inside("-v /var/run/docker.sock:/var/run/docker.sock --group-add '${dockergid}'") {
                     standardSetup()
                     unstash 'omc-clang'