CI: point docker job at dind sidecar via tcp DOCKER_HOST

.gitea/workflows/ci.yml

@@ -27,6 +27,9 @@ jobs:
     if: startsWith(github.ref, 'refs/tags/')
     needs: test
     runs-on: ubuntu-latest
+    env:
+      # dind sidecar listens on tcp; job containers reach it via the docker bridge gateway
+      DOCKER_HOST: tcp://172.17.0.1:2375
     permissions:
       contents: read
       packages: write