Add cache volume and interpolate diff option

docker-compose.yml

@@ -1,6 +1,7 @@
 version: "2"
 volumes:
   pgdata:
+  cache:
 services:
   postgres:
     image: "openmaptiles/postgis:2.3"
@@ -28,12 +29,13 @@ services:
     image: "openmaptiles/import-osm:0.1"
     env_file: .env
     environment:
-      DIFF_MODE: "false"
+      DIFF_MODE: ${DIFF_MODE}
     links:
     - postgres
     volumes:
      - ./data:/import
      - ./build:/mapping
+     - cache:/cache
   import-sql:
     image: "openmaptiles/import-sql:0.1"
     env_file: .env