fix(ci): Use comma-separated build_args for docker-buildx plugin

The docker-buildx plugin expects build_args as a comma-separated string,
not a YAML list. This should fix the build_sha/build_time being null.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.woodpecker/.ci.yml

@@ -64,11 +64,7 @@ steps:
         from_secret: registry_password
       platforms: linux/amd64
       provenance: false
-      build_args:
-        - APP_BUILD_VERSION=${CI_COMMIT_SHA}
-        - APP_GIT_SHA=${CI_COMMIT_SHA}
-        - APP_BUILD_TIME=${CI_PIPELINE_CREATED}
-        - CONTAINER_IMAGE_TAG=${CI_COMMIT_SHA}
+      build_args: APP_BUILD_VERSION=${CI_COMMIT_SHA},APP_GIT_SHA=${CI_COMMIT_SHA},APP_BUILD_TIME=${CI_PIPELINE_CREATED},CONTAINER_IMAGE_TAG=${CI_COMMIT_SHA}
     depends_on: []
     when:
       branch: master