Correct, GitHub is necessary at this point to deploy code.
However, yes, you can ask Disco to fetch an existing Docker image (we use that to self-host RabbitMQ). An example of deploying Meilisearch's image is here [0] with the tutorial here [1].
Do you typically build your Docker images and push them to a registry? Curious to learn more about your deployment process.
[0] https://github.com/letsdiscodev/sample-meilisearch/blob/main...