Fix devcontainer prebuild workflow

Fixed two issues with the devcontainer-prebuild workflow:

1. Added required `runCmd` parameter to devcontainers/ci action
   - The action requires a command to run after building
   - Using simple echo command to validate build success

2. Removed docker/setup-buildx-action step
   - devcontainers/ci action handles Docker setup internally
   - buildx setup was potentially conflicting with the action

These changes should fix the prebuild CI failure that was occurring
when trying to build the dev container image.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: claude

.github/workflows/devcontainer-prebuild.yml

@@ -31,9 +31,6 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v4
 
-      - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
-
       - name: Log in to GitHub Container Registry
         uses: docker/login-action@v3
         with:
@@ -48,6 +45,7 @@ jobs:
           cacheFrom: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
           push: ${{ github.event_name != 'pull_request' && 'always' || 'never' }}
           imageTag: latest
+          runCmd: echo "Dev container built successfully"
 
       - name: Summary
         if: success()