Build multi-platform Docker image

[ftkg]

Adds multi-platform support for amd64 and arm64 by cross-compiling inside the Dockerfile to both architectures.

[ftkg]

Quick steps:

1. Install Docker Desktop.
2. docker buildx create --name mybuilder --driver docker-container --bootstrap --use
3. On nakama/build folder, build, tag and push in a single command:
   docker buildx build "$PWD" --platform linux/amd64,linux/arm64 --file ./Dockerfile --build-arg commit="$(git rev-parse --short HEAD 2>/dev/null)" --build-arg version=2.1.0 -t heroiclabs/nakama:2.1.0 -t heroiclabs/nakama:latest --push

[sesposito]

Superseded by: #1316