Drop custom dind sidecar, use runner-provided docker daemon

The Forgejo runner infra now gives job containers a privileged
container pre-wired to its own dind sidecar via DOCKER_HOST, so
defining our own docker:dind service would start a conflicting
second daemon. Keep the node image + docker CLI install (still
needed) and drop the services/env block.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.forgejo/workflows/build.yml

@@ -9,16 +9,10 @@ jobs:
   build-and-push:
     runs-on: docker
     # actions/checkout & co. are JS actions and need Node, so the job
-    # container has to be a Node image (not a bare docker CLI image). We
+    # container has to be a Node image (not a bare docker CLI image). The
-    # install the Docker CLI into it and talk to a sidecar dind daemon over
+    # runner already provides a privileged container wired up to its own
-    # the network instead of relying on the runner exposing a host socket.
+    # dind sidecar via DOCKER_HOST, so we just need the docker CLI itself.
     container: node:20-bookworm
-    services:
-      docker:
-        image: docker:27-dind
-        options: --privileged
-    env:
-      DOCKER_HOST: tcp://docker:2375
     steps:
       - name: Checkout
         uses: actions/checkout@v4