ci: use base runner + git clone (no Node.js dependency)

Problem: actions/checkout@v4 requires Node.js, but base forgejo/runner:6
(Alpine) doesn't have it. We need to test our Dockerfiles on the bare base image.

Solution:
- runs-on: golang (base Alpine runner with dind docker)
- Replace actions/checkout@v4 with git clone (no Node.js needed)
- Clone to /workspace, run all steps there
- Only push on push events (skip on PR to avoid registry pollution)

This validates that our Dockerfile fixes work correctly on base image.

.gitea/workflows/build-runner-images.yml

@@ -17,22 +17,27 @@ on:
 
 jobs:
   build-runners:
-    runs-on: node
+    runs-on: golang
     env:
       REGISTRY: forgejo.riotpiao.com
       IMAGE_BASE: forgejo.riotpiao.com/rock
     steps:
-      - name: Checkout code
-        uses: actions/checkout@v4
+      - name: Clone repository (base image, no Node.js required)
+        run: |
+          git clone --depth 1 https://forgejo.riotpiao.com/rock/homelab.git /workspace
+          cd /workspace
 
-      - name: Get short SHA
+      - name: Get commit SHA
         id: sha
         run: |
+          cd /workspace
           SHORT_SHA=$(git rev-parse --short HEAD)
           echo "short_sha=${SHORT_SHA}" >> $GITHUB_OUTPUT
+          echo "Got SHA: ${SHORT_SHA}"
 
       - name: Build all runner images (test on PR, push on main)
         run: |
+          cd /workspace
           set -e
           for RUNNER in golang rust node; do
             echo "📦 Building ${RUNNER}-runner..."
@@ -54,7 +59,6 @@ jobs:
             docker push "${IMAGE_BASE}/forgejo-runner-${RUNNER}:latest"
             echo "✅ Pushed ${RUNNER}-runner"
           done
-          echo "✅ All runner images pushed to registry"
         env:
           REGISTRY_USER: ${{ secrets.FORGEJO_REGISTRY_USER }}
           REGISTRY_TOKEN: ${{ secrets.FORGEJO_REGISTRY_TOKEN }}