kong: specify the KONG_PG_HOST environment variable

kong/content.md

@@ -61,6 +61,7 @@ Once the database has been started and prepared, we can start a Kong container a
 $ docker run -d --name kong \
     --link kong-database:kong-database \
     -e "KONG_DATABASE=postgres" \
+    -e "KONG_PG_HOST=kong-database" \
     -e "KONG_CASSANDRA_CONTACT_POINTS=kong-database" \
     -e "KONG_PROXY_ACCESS_LOG=/dev/stdout" \
     -e "KONG_ADMIN_ACCESS_LOG=/dev/stdout" \