Add DOCKER_HOST env var to workflow for k8s DinD support

The k8s runner uses a DinD sidecar accessible via TCP on port 2375.
While the runner daemon config has docker_host set, job containers
don't inherit this - they need the DOCKER_HOST env var set explicitly.

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

.forgejo/workflows/build-container.yaml

@@ -16,6 +16,9 @@ on:
 jobs:
   build:
     runs-on: k8s
+    env:
+      # For k8s runner with DinD sidecar, Docker is available via TCP
+      DOCKER_HOST: tcp://127.0.0.1:2375
     steps:
       - name: Parse tag
         id: parse