apps: Support depoly on push to DOCR.

[andrewsomething]

This is does not include an acceptance test. It is non-trivial to test as it requires interacting with Docker to build and push an image to a DOCR registry. I've tested this, but additional manual verification would be appreciated. Here's an example config:

resource "digitalocean_app" "docr-push-test" {
  spec {
    name = "docr-push-test"
    region = "nyc"

    service {
      name               = "docr-push-test"
      instance_count     = 1
      instance_size_slug = "basic-xxs"

      image {
        deploy_on_push {
          enabled = true
        }
        registry_type = "DOCR"
        repository    = "<image repository name>"
        tag           = "latest"
      }
    }
  }
}

[andrewsomething]

As godo has added generics, the tests won't pass on 1.16 and 1.17. This will need to be merged first: #884