Build for both arm64 & amd64

[lotas]

Currently release process relies on the machine where release is being made:

tc-admin/release.sh

Lines 191 to 192 in 1842472

	docker build -t "taskcluster/tc-admin:${NEW_VERSION}" .
	docker push "taskcluster/tc-admin:${NEW_VERSION}"

Which ends up with releases https://hub.docker.com/r/taskcluster/tc-admin/tags having either linux/amd64 or linux/arm64

We should pick one or build for both