Make use of TARGETARCH and TARGETPLATFORM for better multiarch builds

[dsbaars]

I noticed a build arg was used to define the architecture, while there are TARGETARCH and TARGETPLATFORM variables available.

Now you can build multiarch docker images with the same tag, e.g.:
docker buildx build --platform=linux/arm64,linux/amd64 --build-arg version=dev --file Dockerfile.bullseye . --tag dsbaars/lightning-shell:latest --output "type=registry"

[AaronDewes]

That's a thing in Docker? I never knew that, that makes a lot of stuff so much easier for me, thank you!