Crosscompile docker image for v7

Good article explaining all i know (thanks to @artur.klauser): medium

First start docker with qemu

docker run --rm --privileged multiarch/qemu-user-static --reset -p yes

Second build for the targeted platform and push to dockerhub

docker buildx build -t "darkinito/snapserver:latest"  --platform linux/amd64,linux/arm64,linux/arm/v7,linux/arm/v6 --push .