Wrong canonicalWebUrl written by centos image

Change-Id: I2d4ca3d39e03b14d2ab980aae11826d167a418e9
diff --git a/centos/7/Dockerfile b/centos/7/Dockerfile
index 8ce4214..12eb863 100644
--- a/centos/7/Dockerfile
+++ b/centos/7/Dockerfile
@@ -16,12 +16,14 @@
 RUN java -jar /var/gerrit/bin/gerrit.war init --batch --install-all-plugins -d /var/gerrit
 RUN java -jar /var/gerrit/bin/gerrit.war reindex -d /var/gerrit
 
+ENV CANONICAL_WEB_URL=
+
 # 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://$HOSTNAME:8080/ && \
+CMD git config -f /var/gerrit/etc/gerrit.config gerrit.canonicalWebUrl "${CANONICAL_WEB_URL:-http://$HOSTNAME:8080/}" && \
     git config -f /var/gerrit/etc/gerrit.config noteDb.changes.autoMigrate true && \
-    /var/gerrit/bin/gerrit.sh run
+        /var/gerrit/bin/gerrit.sh run
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