Store backend ci result

.drone.yml

@@ -6,14 +6,24 @@ name: Build backend
 steps:
   - name: Build backend
     image: nixos/nix
+    volumes:
+      - name: cache
+        path: /drone-shared
     commands:
       - cd backend
       - nix-channel --update
       - nix-build
+      - cp result /drone-shared/jitsi-rooms-docker-image
 trigger:
   event:
     include:
       - tag
+      - push
+
+volumes
+- name: drone-shared
+  host:
+    path: /tmp/drone-shared
 
 ---
 kind: pipeline