Create a DigitalOcean Container Registry via API, authenticate docker, push an image, and reclaim space

Documented steps

1. POST https://api.digitalocean.com/v2/registries with {"name":"my-registry","subscription_tier_slug":"starter|basic|professional","region":"<optional-slug>"}. The name must be globally unique across all DigitalOcean registries, lowercase alphanumeric/dashes.
2. Prefer the plural /v2/registries endpoints: the legacy singular /v2/registry API predates multi-registry support and some of its endpoints don't work when an account has more than one registry (docs: https://docs.digitalocean.com/products/container-registry/reference/api/).
3. Authenticate docker: easiest is `doctl registry login` (uses your API token). Docs also show direct `docker login registry.digitalocean.com` using your DigitalOcean API token as the password (see https://docs.digitalocean.com/products/container-registry/how-to/use-registry-docker-kubernetes/ for the exact username convention). A docker-credentials API endpoint can mint read-only or read_write config.json credentials.
4. Tag and push: `docker tag my-app:latest registry.digitalocean.com/<registry-name>/my-app:v1` then `docker push registry.digitalocean.com/<registry-name>/my-app:v1`.
5. Deleting tags does not free storage by itself — run garbage collection (control panel or the registry garbage-collection API) to delete unreferenced blobs and reclaim billed space.