Enhance Docker CLI setup in workflow: resolve DOCKER_HOST dynamically and update README

.forgejo/workflows/publish-container.yml

@@ -16,9 +16,8 @@ jobs:
   publish:
     runs-on: docker
     env:
-      # Keep the runner-provided DOCKER_HOST when available.
-      # For common dind sidecar setups, this default works out of the box.
-      DOCKER_HOST: ${{ vars.DOCKER_HOST != '' && vars.DOCKER_HOST || 'tcp://forgejo-docker-in-docker:2375' }}
+      # Allow explicit override from Forgejo variables.
+      DOCKER_HOST: ${{ vars.DOCKER_HOST }}
 
     steps:
       - name: Checkout
@@ -66,6 +65,37 @@ jobs:
           echo "${HOME}/.local/bin" >> "${GITHUB_PATH}"
           "${HOME}/.local/bin/docker" --version
 
+      - name: Resolve Docker daemon endpoint
+        run: |
+          set -eu
+
+          if [ -n "${DOCKER_HOST:-}" ]; then
+            echo "Using configured DOCKER_HOST=${DOCKER_HOST}"
+            exit 0
+          fi
+
+          if [ -S /var/run/docker.sock ]; then
+            echo "DOCKER_HOST=unix:///var/run/docker.sock" >> "${GITHUB_ENV}"
+            echo "Resolved DOCKER_HOST from local docker socket"
+            exit 0
+          fi
+
+          if getent hosts forgejo-docker-in-docker >/dev/null 2>&1; then
+            echo "DOCKER_HOST=tcp://forgejo-docker-in-docker:2375" >> "${GITHUB_ENV}"
+            echo "Resolved DOCKER_HOST from compose service DNS"
+            exit 0
+          fi
+
+          GATEWAY_IP="$(ip route | awk '/default/ { print $3; exit }')"
+          if [ -n "${GATEWAY_IP}" ]; then
+            echo "DOCKER_HOST=tcp://${GATEWAY_IP}:2375" >> "${GITHUB_ENV}"
+            echo "Resolved DOCKER_HOST from container default gateway: ${GATEWAY_IP}"
+            exit 0
+          fi
+
+          echo "Could not determine a reachable Docker daemon endpoint. Set vars.DOCKER_HOST explicitly."
+          exit 1
+
       - name: Check Docker daemon connectivity
         run: |
           set -eu