upgrade-model on k8s fails with invalid response code from registry: 404

[George Kraft]

I have a Juju controller that was bootstrapped with:

--config caas-image-repo=rocks.canonical.com/cdk/jujusolutions

When trying to upgrade a Kubernetes model from Juju version 2.8.11 to 2.9.12, the command fails with:

$ juju upgrade-model
ERROR invalid response code from registry: 404

When run with --debug, it appears to make a request with a malformed URL to find jujud-operator image tags:

$ juju upgrade-model --debug
12:13:36 INFO juju.cmd supercommand.go:56 running juju [2.9.12 0 86d7f5193303a307171fba4cea7745b326023282 gc go1.14.15]
...
12:13:38 DEBUG juju.docker docker.go:40 operater image tags URL: https://registry.hub.docker.com/v1/repositories/rocks.canonical.com/cdk/jujusolutions/jujud-operator/tags
12:13:39 DEBUG juju.api monitor.go:35 RPC connection died
12:13:39 DEBUG juju.api monitor.go:35 RPC connection died
12:13:39 DEBUG juju.api monitor.go:35 RPC connection died
ERROR invalid response code from registry: 404
12:13:39 DEBUG cmd supercommand.go:537 error stack:
/build/snapcraft-juju-35d6cf/parts/juju/src/docker/docker.go:87: invalid response code from registry: 404
/build/snapcraft-juju-35d6cf/parts/juju/src/docker/docker.go:43:
/build/snapcraft-juju-35d6cf/parts/juju/src/cmd/juju/commands/upgrademodel.go:815:

Given that the caas-image-repo config points to a different registry, I do not think it should be reaching out to registry.hub.docker.com at all, and certainly not with that malformed URL.