Docker image: add support for arm64

[Martin-happy]

Chore summary
AFAI can see there is no docker-image available for linux/amd64.
Running spectral lint in our cicd pipeline (in docker) hits a blocker when the job runs in an arm64 arch node.

Tasks

• extend docker build process to also build and push image for arm64 and potential others

Additional context
docker multi arch build: https://www.docker.com/blog/multi-arch-build-and-images-the-simple-way/
current available tags on dockerhub: https://hub.docker.com/r/stoplight/spectral/tags

Docker trying to auto-match an image for the host platform on an linux/arm64/v8 machine:

Status: Downloaded newer image for stoplight/spectral:latest
WARNING: The requested image's platform (linux/amd64) does not match the detected host platform (linux/arm64/v8) and no specific platform was requested
exec /usr/local/bin/spectral: exec format error

[Martin-happy]

I think this could be a fast win as the arm64 build in general is already set up
#2569