Switch from docker build to buildx for CI/CD.
- The main workflow file has been optimised and simplified.
- Images are built in parallel when building locally resulting in faster build times.
- The github action workflow is about 50% faster.
- Arm images are built as well. These images are not tested due to restrictions of github actions (no arm runners). The tags of the images have -arm appended to it.
- Arm images can also be built locally.
- Reusable workflow is introduced for building, testing and deploying the images.
  This allows the workflow to be reused for other purposes in the future.
- Workflow can be manually triggered. This allows forked Mailu projects to also use the workflow for building images.