Add Multi-Architecture Docker Build Support

Update the Docker build workflow to support multi-architecture builds (AMD64 and ARM64) for better compatibility with AWS Graviton instances and Apple Silicon development environments.

## Benefits
* **Cost Optimization**: Enable deployment on AWS Graviton instances (up to 40% better price-performance)
* **Developer Experience**: Support Apple Silicon Macs (M1/M2/M3) for local development
* **Future-Proofing**: ARM64 adoption is increasing across cloud providers

## Implementation
The updated workflow will:
* Use Docker Buildx for cross-platform builds
* Build for `linux/amd64` and `linux/arm64` platforms
* Push multi-arch manifest to ECR
* Maintain backward compatibility with existing x86 deployments

## Testing
* [ ] Verify builds complete successfully for both architectures
* [ ] Test deployment on x86 EC2 instances (existing functionality)
* [ ] Test deployment on Graviton EC2 instances (new functionality)
* [ ] Confirm local development works on Apple Silicon

Start by locating the Docker build workflow and its current ECR publishing step, then inspect how the existing x86 image is built and pushed. Done means successful AMD64 and ARM64 builds, a multi-architecture ECR manifest, and verified deployment on both x86 EC2 and Graviton instances, plus local development on Apple Silicon.

Written by the indexing model from the issue text.