Feature: publish multi-architecture Docker images

[ocanema]

Problem

DrogonSec Docker images are currently built as linux/amd64-only artifacts. That makes Docker usage less convenient on arm64 hosts such as Apple Silicon and ARM Linux runners, where users may need emulation or separate local builds.

Proposed solution

Update the Docker build pipeline to publish multi-platform images for linux/amd64 and linux/arm64:

• Configure QEMU before Docker Buildx in CI and staging workflows.
• Pass linux/amd64,linux/arm64 to docker/build-push-action.
• Let the Dockerfile build stage use BuildKit target platform args instead of hard-coded GOOS/GOARCH values.
• Add Makefile targets/variables for local multi-arch Docker builds and pushes.

Alternatives considered

• Keep publishing amd64-only images and require arm64 users to build locally.
• Publish separate architecture-specific tags instead of a single manifest list.
• Only update the Dockerfile without changing CI, which would not publish multi-arch images automatically.

[filipi86]

Thanks for the proposal, @ocanema — multi-arch images (arm64 / Apple Silicon) make sense. We're evaluating the implementation in #42, including a security review of the workflow changes, and will follow up there. 🙌