Added support for enabling kiosk mode and opening an URL at launch.

appdefs.yml

@@ -261,7 +261,19 @@ container:
   unsupported_volume: /storage
 
   # Environment variables.
-  environment_variables: []
+  environment_variables:
+    - name: FF_OPEN_URL
+      description: >-
+        The URL to open when {{ app.friendly_name }} starts.
+      type: public
+      default:
+    - name: FF_KIOSK
+      description: >-
+        Set to `1` to enable kiosk mode.  This mode launches Firefox in a very
+        restricted and limited mode best suitable for public areas or
+        customer-facing displays.
+      type: public
+      default: 0
 
   # Volumes
   volumes: []