Add Docker ARM64 support (and how to do it)

Hey there, thanks for making this software.

My lemmy instance (https://lemmy.emerald.show) is running on an ARM server. While lcs Docker container doesn't support ARM, it's easy to make it so.

A demo image (made from commit `8ae9a6e350b0334f785619b98cef0548efec3863`) is here: https://hub.docker.com/r/peterwilli/lcs/tags

If you wish to add official support:

- Install the binfmt emulator (emulates ARM on X86 build workers): `docker run --privileged --rm tonistiigi/binfmt --install all`
- Run `docker buildx create --use`
- In the lcs source repo, run `docker buildx build --tag=/lcs --platform linux/arm64,linux/amd64 --push .`

Hope this helps! This builds both amd64 and arm64.

I tested it on:

- Odroid M1 server
- Rockchip RK2568B2 quad-core ARM Cortex-A55
- 8GB RAM