optimize CI pipeline (run *only* on changes/tags)

.gitlab-ci.yml

@@ -43,6 +43,12 @@ build:backend:
     paths:
       - target/release/upend
     expire_in: 1 day
+  only:
+    changes:
+      - migrations/**/*
+      - src/**/*
+      - Cargo.*
+      - Makefile
 
 build:frontend:
   stage: build
@@ -54,6 +60,10 @@ build:frontend:
     paths:
       - ui/dist
     expire_in: 1 day
+  only:
+    changes:
+      - ui/**/*
+      - Makefile
 
 test:backend:
   stage: test
@@ -76,3 +86,5 @@ package:
   artifacts:
     paths:
       - ./*.AppImage
+  only:
+    - tags