Update Gerrit config for quickstart

The entrypoint script in the Gerrit image has changed.  This is the
new way to specify the canonical URL.  The other entries we were
setting are either no longer necessary or not critical.

Change-Id: I6d8979a5fdb204b9a3a12acf05a56a13413379c1

doc/source/examples/docker-compose.yaml

@@ -8,13 +8,8 @@ services:
     ports:
       - "8080:8080"
       - "29418:29418"
-    command: |
-      /bin/sh -c '\
-        git config -f /var/gerrit/etc/gerrit.config gerrit.canonicalWebUrl http://localhost:8080/ && \
-        git config -f /var/gerrit/etc/gerrit.config gerrit.ui POLYGERRIT && \
-        git config -f /var/gerrit/etc/gerrit.config sendemail.enable false && \
-        git config -f /var/gerrit/etc/gerrit.config noteDb.changes.autoMigrate true && \
-        /var/gerrit/bin/gerrit.sh run'
+    environment:
+      - CANONICAL_WEB_URL=http://localhost:8080/
   gerritconfig:
     image: zuul/zuul-executor
     environment: