Dockerfile: update buildx to v0.12.1

Update the version of buildx used in CI to the latest version.

- full diff: https://github.com/docker/buildx/compare/v0.12.0...v0.12.1
- release notes: https://github.com/docker/buildx/releases/tag/v0.12.1

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>