ci: use docker:cli container override for image-pull job

- Replace static docker binary download with container: image: docker:cli
- Remove act-runner labels config (default already uses proper Ubuntu image)
- Remove CONFIG_FILE env var (no longer needed)

.gitea/workflows/ci.yml

@@ -58,6 +58,11 @@ jobs:
     runs-on: ubuntu-latest
     # Only run on PRs that touch nomad job specs
     if: github.event_name == 'pull_request'
+    # Use the official Docker CLI image so we get a versioned, maintained docker
+    # binary without hardcoding a static download URL. The runner's docker socket
+    # is already mounted by act_runner, so docker commands work out of the box.
+    container:
+      image: docker:cli
 
     steps:
       - uses: actions/checkout@v4
@@ -66,8 +71,6 @@ jobs:
 
       - name: Pull changed images
         run: |
-          curl -fsSL https://download.docker.com/linux/static/stable/x86_64/docker-27.5.1.tgz \
-            | tar -xz --strip-components=1 -C /usr/local/bin docker/docker
           git fetch origin ${{ github.base_ref }}
           IMAGES=$(git diff origin/${{ github.base_ref }}...HEAD -- '*.nomad.hcl' \
             | grep '^+\s*image\s*=' \