Use canonical image names when adding to the cache

[afbjorklund]

Currently, we save the images as archives using only a short name such as "busybox".

[
  {
    "Config": "sha256:388056c9a6838deea3792e8f00705b35b439cf57b3c9c2634fb4e95cfc896de6",
    "RepoTags": [
      "busybox"
    ],
    "Layers": [
      "f531cdc67389c92deac44e019e7a1b6fba90d1aaa58ae3e8192f0e0eed747152.tar.gz"
    ]
  }
]

We should make sure to use the canonical name, e.g. "docker.io/library/busybox:latest"

[
  {
    "Config": "sha256:388056c9a6838deea3792e8f00705b35b439cf57b3c9c2634fb4e95cfc896de6",
    "RepoTags": [
      "docker.io/library/busybox:latest"
    ],
    "Layers": [
      "f531cdc67389c92deac44e019e7a1b6fba90d1aaa58ae3e8192f0e0eed747152.tar.gz"
    ]
  }
]

The go-container-registry library can do this (Name), only have to work around index.docker.io

var (
	legacyDefaultDomain = "index.docker.io"
	defaultDomain       = "docker.io"
	officialRepoName    = "library"
	defaultTag          = "latest"
)

       // Rewrite "docker.io" to "index.docker.io".
        // See: https://github.com/google/go-containerregistry/issues/68
        if name == defaultRegistryAlias {
                name = DefaultRegistry
        }

Having the canonical name means that we can load them into container runtimes such as cri-o.

The library calls this "strict validation", as opposed to "weak validation" (without registry and tag)

---

Eventually it also opens up for having a different default registry.

/ WithDefaultRegistry sets the default registry that will be used if one is not
// provided.
func WithDefaultRegistry(r string) Option

But in the short term, it fixes some shortcomings with the cache: #8554

See also https://www.redhat.com/sysadmin/container-image-short-names

We can use the same short name as argument and as a cache filename.