Add option to push multiarch Docker images

[timlapluie]

You can now provide an additional option to the push_image action in order to push multiarch (x86_64 and arm64) Docker images into AWS ECR. You can use it like this:

- sage/sbc-shared#multiarch-image-push:
    action: push_image
    environment: qa
    landscape: eu
    account_id: "123123123123"
    region: "eu-west-1"
    db_image: true
    multiarch_image_push: true

The only thing you have to make sure is that before running the push_image action, you have to build an additional arm64 Docker image (with the tag suffix arm64).