Docker image only builds for amd64; add arm64 support

## Problem The Docker workflow (`.github/workflows/docker.yml`) builds and publishes images only for `linux/amd64`. QEMU and Buildx are already configured in the CI but the `platforms` field is limited to a single architecture. This means `docker pull ghcr.io/gyulyvgc/sniffnet:latest` fails or falls back to emulation on arm64 machines (Apple Silicon, AWS Graviton, Raspberry Pi, etc.), despite the project already distributing arm64 DEB/RPM/AppImage packages. ## Proposed Solution Add `linux/arm64` to the `platforms` list in `docker/build-push-action`. The CI infrastructure is already in place (`setup-qemu-action` + `setup-buildx-action`), and both `rust:1.88-slim` and `debian:bookworm-slim` are multi-arch base images. ## Change ```diff - platforms: linux/amd64 + platforms: linux/amd64,linux/arm64 ```