[CI] Fix drone secrets usage

.drone.yml

@@ -24,9 +24,13 @@ steps:
   image: docker:stable
   environment:
     DOCKER_HOST: tcp://docker:2375
+    USERNAME:
+      from_secret: dockerhub_username
+    PASSWORD:
+      from_secret: dockerhub_password
   commands:
   - docker build -t kleph/beer .
-  - docker login -u $DOCKERHUB_USERNAME -p $DOCKERHUB_PASS
+  - docker login -u $USERNAME -p $PASSWORD
   - docker image push kleph/beer
 
 services: