CI: switch to docker buildx and enable arm64 docker images

[Manawyrm]

Hi,

for modern cloud servers and single board computers (like the popular Raspberry Pi's) it would be really nice to have arm64 images available.

This PR switches the docker build process over to Dockers new buildx process, which handles multi-arch building of images automatically.

The images at Dockerhub (https://hub.docker.com/r/rofl256/whiteboard ) don't seem to be generated from GitHub Actions yet (probably manually generated?).

There is one downside to doing this: arm64 build is done using QEMU emulation, which is a bit slower. An alternative way would be to use GitHub's native ARM64 runners for this, but it's more setup work.

I have tested this container on ARM64 and it seems to work very well:

What do you think?

[cracker0dks]

thanks a lot 👍 yeah the image on dockerhub is auto generated by a server script and pushed.

[Manawyrm]

Cool! Thanks for merging :)