fix image tag

.drone.yml

@@ -9,10 +9,10 @@ steps:
   commands:
   - |
     if  [[ "$DRONE_BRANCH" == "master" ]]; then
-      tag="";
+      tag="latest";
       echo "Running on default branch '$DRONE_BRANCH': tag = 'latest'";
     else
-      tag=":$DRONE_BRANCH";
+      tag="$DRONE_BRANCH";
       echo "Running on branch '$DRONE_BRANCH': tag = $tag";
     fi
   - echo "tag=$tag" >> .env