add k8s docker group to jenkins user

Change-Id: I160b9dbc6a38d90b3f3cf9c79bca786d48059afb

jenkins/Dockerfile

@@ -33,6 +33,7 @@ ARG user=jenkins
 ARG group=jenkins
 ARG uid=1000
 ARG gid=1000
+ARG docker_gid=994
 ARG http_port=8080
 ARG agent_port=50000
 
@@ -44,7 +45,7 @@ ENV JENKINS_SLAVE_AGENT_PORT ${agent_port}
 # ensure you use the same uid
 RUN groupadd -g ${gid} ${group} \
     && useradd -d "$JENKINS_HOME" -u ${uid} -g ${gid} -m -s /bin/bash ${user} \
-    && usermod -aG docker ${user}
+    && echo "docker:x:${docker_gid}:${user}" >> /etc/group
 
 # Jenkins home directory is a volume, so configuration and build history
 # can be persisted and survive image upgrades