Follow the Quickstart instructions for creating a DigitalOcean Container Registry, and push your image to the registry.
When updating the source image tag, a new deployment will be triggered. If the actual tag name has not changed, it is not guaranteed to pull down a fresh copy of the image due to caching. The recommended approach is to use unique tag names for all image updates.
You can change the tag of the container image to use as the source of an existing deployment: