Enhance Docker Build Script for Compatibility with Containerd Layer Store

## Problem Description
The recent introduction of the `containerd` layer store in Depot-hosted runners exposed gaps in our understanding and configuration of Docker’s build process. The feature is working as designed, but it conflicts with our existing workflow when handling [multi-architecture builds](https://docs.docker.com/build/building/multi-platform/) and provenance data.

Specifically:
- Single-architecture images inadvertently include manifest lists with an unexpected "unknown" platform due to Docker's default build provenance metadata.
- The legacy Docker layer store previously stripped this metadata, but the `containerd` store natively supports manifest lists and does not modify the image format.
- Modifying our scripts is necessary to fully support the `containerd` layer store while retaining provenance data and enabling multi-architecture builds.

## Desired Solution
1. **Update `build_docker.sh` to Use `docker buildx`:**
- Incorporate `docker buildx build` to handle [multi-platform builds](https://docs.docker.com/build/building/multi-platform/) efficiently.
- Ensure proper handling of provenance metadata and manifest merging when creating multi-arch images.

2. **Integrate `depot build` as an Optional Workflow:**
- Add a flag to toggle between `docker buildx` and `depot build` for CI environments.
- Use `depot build` for efficient multi-arch builds where possible, falling back to `docker buildx` for broader compatibility.

3. **Enhance CI Workflow:**
- Default to `docker buildx` for local and dev workflows to avoid additional dependencies.
- Enable `depot build` only in CI and set it up dynamically using a GitHub Action.

4. **Optimize for Single-Arch Builds:**
- Modify the script to build single-architecture images efficiently (e.g., during dogfood development) while keeping multi-arch workflows intact in CI.

## Next Steps
- Update `scripts/build_docker.sh` to support both `docker buildx` for local dev and `depot build` for ci.
- Test single-architecture and multi-architecture builds using both approaches to ensure functionality with the `containerd` layer store.

## References
- [Docker Buildx Documentation](https://docs.docker.com/buildx/working-with-buildx/)
- [Docker Provenance Metadata](https://docs.docker.com/build/metadata/attestations/slsa-provenance/)
- [`containerd` image store](https://docs.docker.com/desktop/features/containerd/)
- [Multi-platform builds](https://docs.docker.com/build/building/multi-platform/)
- [Slack Thread](https://codercom.slack.com/archives/C07AL9CP7B2/p1734075806309169)

This issue will help us fully leverage the `containerd` layer store's benefits while maintaining compatibility and development flexibility.