terraform-provider-docker/examples/resources/docker_image/resource-build-triggers.tf

resource "docker_image" "zoo" {
  name = "zoo"
  build {
    context = "."
  }
  triggers = {
    dir_sha1 = sha1(join("", [for f in fileset(path.module, "src/**") : filesha1(f)]))
  }
}