How can I use Docker Hub in Terraform?
This can be done with docker_registry_image:

data "docker_registry_image" "ubuntu" {
  name = "ubuntu:latest"
}

resource "docker_image" "ubuntu" {
  name          = "${data.docker_registry_image.ubuntu.name}"
  pull_triggers = ["${data.docker_registry_image.ubuntu.sha256_digest}"]
}