Add multi-architecture support to devcontainer build (#14)

## Summary
Build Docker image for both `linux/amd64` and `linux/arm64` to support
ARM64 Macs (M1/M4) and x86_64 systems.

The workflow was only building for the runner's architecture (x86_64).
This change enables multi-platform builds so the devcontainer image
works on both ARM64 Macs and x86_64 machines.

## Test plan
- [ ] Workflow runs and builds successfully for both architectures
- [ ] Image can be pulled on ARM64 Mac

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

Co-authored-by: Claude Haiku 4.5 <noreply@anthropic.com>

Author: jverkoey

.github/workflows/build-and-push.yml

@@ -27,6 +27,9 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v4
 
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
       - name: Log in to the Container registry
         uses: docker/login-action@v3
         with:
@@ -55,3 +58,4 @@ jobs:
           push: ${{ github.event_name != 'pull_request' }}
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
+          platforms: linux/amd64,linux/arm64