resiliency: switch from hub.docker.io to ghcr to avoid HTTP 401 Not Authorized failures

[ssbarnea]

As hub.docker.io has rate limits ensuring that the action uses ghcr makes is far less likely to fail. I spotted multiple failures recently like here:

ERROR: unexpected status from HEAD request to https://registry-1.docker.io/v2/library/alpine/manifests/3.12: 401 Unauthorized

https://github.com/sourcegraph/codenotify/blob/main/Dockerfile#L1 should be modified to rely on an image hosted on ghcr.

What makes the issue even worse is that continue-on-error: true does not prevent the job from being marked as failure in this case, so I am forced to temporary stop using the action.