Use vars in the env (oooh, does it work!?)

deploy.d/config.yml

@@ -1,4 +1,7 @@
 ---
+vars:
+  port: "8042"
+
 volumes:
   - type: directory
     src: /var/lib/drone
@@ -16,6 +19,6 @@ env:
   DRONE_JSONNET_ENABLED: "true"
   DRONE_RPC_SECRET: "{{ vault.rpc_secret }}"
   DRONE_SERVER_HOST: "drone.autonomic.zone"
-  DRONE_SERVER_PORT: ":8042"
+  DRONE_SERVER_PORT: ":{{ vars.port }}"
   DRONE_SERVER_PROTO: "https"
   DRONE_USER_FILTER: "autonomic-cooperative"