ci: support tag-triggered pipelines with versioned Docker images

- Allow triggering on v* tags in addition to pushes to main/master
- Tag Docker images with CI_COMMIT_TAG when triggered by a tag

.woodpecker/build.yml

@@ -1,6 +1,8 @@
 when:
   - event: push
     branch: [main, master]
+  - event: tag
+    tags: [v*]
 
 steps:
   install-deps:
@@ -19,7 +21,7 @@ steps:
     image: docker
     commands:
       - docker build -t likedream-web:latest .
-      - docker tag likedream-web:latest likedream-web:${CI_COMMIT_SHA:0:8}
+      - docker tag likedream-web:latest likedream-web:${CI_COMMIT_TAG:-${CI_COMMIT_SHA:0:8}}
     depends_on:
       - build-frontend
     volumes:
@@ -47,6 +49,3 @@ steps:
       - build-image
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock
-    when:
-      event: [push, tag]
-      branch: [main, master]