chore: introduce multiplatform build

[miraclx]

Context: #194

Following #212, this introduces arm64 and armv7 builds of the docker image.

https://docs.docker.com/desktop/multi-arch/#build-multi-arch-images-with-buildx

To pull the image for this PR:

$ docker pull freyrcli/freyrjs-git:pr-214

Image metadata:

$ docker buildx imagetools inspect freyrcli/freyrjs-git:pr-214
Name:      docker.io/freyrcli/freyrjs-git:pr-214
MediaType: application/vnd.docker.distribution.manifest.list.v2+json
Digest:    sha256:be39922ad30fccc2fa43f1ffd7247b7f8f62a1d7cc2ceed99b063dac3a0e97a8
           
Manifests: 
  Name:      docker.io/freyrcli/freyrjs-git:pr-214@sha256:d3da440dfeca9016effe1f4b6251d62a50c0c938e8a7a80e3f103bb0de69f2d5
  MediaType: application/vnd.docker.distribution.manifest.v2+json
  Platform:  linux/amd64
             
  Name:      docker.io/freyrcli/freyrjs-git:pr-214@sha256:8bb7f00f59f56b1590cd5ab30b157e40f80dd63f9bf82cd7bea83ef2162269fe
  MediaType: application/vnd.docker.distribution.manifest.v2+json
  Platform:  linux/arm64
             
  Name:      docker.io/freyrcli/freyrjs-git:pr-214@sha256:4d6cfb8881b46a2640961932162dcfad782ab1ce0a4a8b53c1292568e0cbf3b4
  MediaType: application/vnd.docker.distribution.manifest.v2+json
  Platform:  linux/arm/v7