dockerised_local_env: Make gerrit-1 ssh host known

Before this change, gerrit-2's initial replication command towards
gerrit-1 caused unknown ssh host warnings in the docker log.

Bug: Issue 12492
Change-Id: I574fa1fe555ba284eb633c67c7b01109f4440e91

dockerised_local_env/gerrit-2/docker-entrypoint.sh

diff --git a/dockerised_local_env/gerrit-2/docker-entrypoint.sh b/dockerised_local_env/gerrit-2/docker-entrypoint.sh
index 532377c..e13f768 100755
--- a/dockerised_local_env/gerrit-2/docker-entrypoint.sh
+++ b/dockerised_local_env/gerrit-2/docker-entrypoint.sh
@@ -11,6 +11,7 @@
 
   echo "Waiting for gerrit1 server to become available."
   sleep 120
+  ssh-keyscan -t rsa -p 29418 gerrit-1 > /var/gerrit/.ssh/known_hosts
   ssh -p 29418 admin@gerrit-1 replication start
   echo "Waiting for replication to complete."
   sleep 30