ci: dump docker socket diagnostics when the daemon poll times out

The 30s poll is enough to ride out a slow daemon startup, but not a
runner where the socket isn't mounted at all. Print DOCKER_HOST, the
socket stat, and the verbose `docker info` error on timeout so the
next flaky run tells us whether it's a startup race or a missing
mount.

.forgejo/workflows/build.yaml

@@ -51,12 +51,25 @@ jobs:
           # `docker info` until it responds so the login/build steps
           # below don't race the socket and fail with "unable to
           # connect to docker".
+          sock="${DOCKER_HOST:-unix:///var/run/docker.sock}"
+          echo "Using DOCKER_HOST=${sock}"
+
           for i in $(seq 1 30); do
-            docker info >/dev/null 2>&1 && exit 0
+            if docker info >/dev/null 2>&1; then
+              echo "Docker daemon reachable after ${i}s"
+              exit 0
+            fi
             echo "Waiting for Docker daemon... ($i/30)"
             sleep 1
           done
-          echo "Docker daemon never became reachable" >&2
+
+          echo "Docker daemon never became reachable — diagnostics:" >&2
+          echo "--- ls -l /var/run/docker.sock ---" >&2
+          ls -l /var/run/docker.sock 2>&1 || true
+          echo "--- env | grep -i docker ---" >&2
+          env | grep -i docker 2>&1 || true
+          echo "--- docker info (verbose) ---" >&2
+          docker info 2>&1 || true
           exit 1
 
       - name: Generate image tag