Enable persistence of external data in Docker

When using a Docker-based deployment for production, allow
to mount external volumes to load configuration and persist
data.

Change-Id: Iad8bf2c576b70d9652463980859953f248c4b93b
diff --git a/docker/centos7/Makefile b/docker/centos7/Makefile
index 631a666..156dbdb 100644
--- a/docker/centos7/Makefile
+++ b/docker/centos7/Makefile
@@ -34,6 +34,8 @@
 # Allow incoming traffic
 EXPOSE 29418 8080
 
+VOLUME ["/var/gerrit/git", "/var/gerrit/index", "/var/gerrit/cache", "/var/gerrit/db", "/var/gerrit/etc"]
+
 # Start Gerrit
 CMD git config -f /var/gerrit/etc/gerrit.config gerrit.canonicalWebUrl http://$$HOSTNAME:8080/ && /var/gerrit/bin/gerrit.sh start && tail -f /var/gerrit/logs/error_log
 endef
diff --git a/docker/ubuntu16.04/Makefile b/docker/ubuntu16.04/Makefile
index b216b74..08c5b04 100644
--- a/docker/ubuntu16.04/Makefile
+++ b/docker/ubuntu16.04/Makefile
@@ -35,6 +35,8 @@
 # Allow incoming traffic
 EXPOSE 29418 8080
 
+VOLUME ["/var/gerrit/git", "/var/gerrit/index", "/var/gerrit/cache", "/var/gerrit/db", "/var/gerrit/etc"]
+
 # Start Gerrit
 CMD git config -f /var/gerrit/etc/gerrit.config gerrit.canonicalWebUrl http://$$HOSTNAME:8080/ && /var/gerrit/bin/gerrit.sh start && tail -f /var/gerrit/logs/error_log
 endef