fix: use map format for docker-buildx build_args (#59)

The woodpecker docker-buildx plugin expects build_args as a YAML map (KEY: VALUE), not a list (- KEY=VALUE). The list format was silently ignored, so BASE_PATH was never passed to the Docker build.

Reviewed-on: #59
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

.woodpecker/docker.yaml

@@ -23,7 +23,7 @@ steps:
       dockerfile: ui/Dockerfile.ui
       context: ui
       build_args:
-        - BASE_PATH=/ui
+        BASE_PATH: /ui
       username: droneci
       password:
         from_secret: DRONECI_PASSWORD