Use GitHub Actions for multi-arch Docker images

[Almighty-Alpaca]

You can see an example of this in action here and the resulting images here.

With the current configuration images are build for the following architectures: linux/amd64, linux/386, linux/arm/v7, linux/arm64. This will allow people to run Galactus on their Raspberry Pis. More architectures can be added, but they will increase build times (and are probably not really useful).

To use this you need to configure three secrets in your repository:

• DOCKERHUB_REPOSITORY: The repository name to push to, e.g. automuteus/galactus
• DOCKERHUB_USERNAME: Username of your Docker Hub account
• DOCKERHUB_TOKEN: Docker Hub access token (can be generated here)

If this gets approved I plan to give the AutoMuteUs bot itself the same treatment so that the whole stack is built for ARM devices.

[denverquane]

Nice, looks good! Appreciate it!