[Feature Request] Add docker hub as a project backend #1973
Description
It'd be very beneficial to my workflows if Docker Hub was added as a backend to check for new versions.
Some projects only publish their releases to Docker Hub, but others (such as Nextcloud) publish the container for their releases some time (often a day or more) after the release is published on Github. This makes relying on the github release unreliable for container deployments as the release will be detected (and update automation triggered) before the actual distribution package is available.
I'd like to add support for using Docker Hub as a source for releases. This would require a way to provide credentials to the Anitya application at runtime as the docker hub API requires authentication for most endpoints. I imagine the easiest way to do this is via environment variables, but a more preferred method could be used instead. I haven't found any examples of other backends that require authentication so I'm not sure if there is existing precedent to draw on here.
If this is acceptable, I can take on implementing this and submitting a PR.