Fix Gerrit configuration in gerrit-master start-script

In the start script of the gerrit-master docker image, the addition of
a javaOption failed, because it was a multi-value option and the '--add'
flag wasn't used. This change fixes that issue.

Change-Id: I38eb8bdd7a230d749bf47129c1f6a5fbf02345c2
diff --git a/container-images/gerrit-master/tools/start b/container-images/gerrit-master/tools/start
index 764a34e..9e2942e 100755
--- a/container-images/gerrit-master/tools/start
+++ b/container-images/gerrit-master/tools/start
@@ -18,7 +18,8 @@
 java -jar /var/gerrit/bin/gerrit.war reindex \
     -d /var/gerrit
 
-git config -f /var/gerrit/etc/gerrit.config container.javaOptions "-Djava.security.egd=file:/dev/./urandom"
+git config -f /var/gerrit/etc/gerrit.config --add \
+    container.javaOptions "-Djava.security.egd=file:/dev/./urandom"
 
 /var/gerrit/bin/gerrit.sh start