No ability to add docker.io credentials in custom_registries

[Bartosz Woronicz]

The charm does no handle properly when we put docker.io in custom_registries,
for instance
juju config custom_registries='[{"host": "docker.io", "username": "testtux", "password": "somepassword"}]'

That will result in fault config.toml with doubled mirror entries for docker.io
```
$ journalctl -u containerd -f
...
Sep 16 10:41:56 hpc-k8s-vm-mcp-1 containerd[540735]: containerd: failed to load TOML: /etc/containerd/config.toml: (53, 6): duplicated tables
```

```
    [plugins.cri.registry]
      [plugins.cri.registry.mirrors]
        [plugins.cri.registry.mirrors."docker.io"]
          endpoint = ["https://registry-1.docker.io"]
    [plugins.cri.registry.mirrors."docker.io"]
```

The fix for that would be check the custom_registries list if it doesn't contain the docker.io already.

That consider charm versions up to these commits
```

commit 9f15599353613d2f3e8a0d120ba06c3dd0f5a281 (HEAD -> refs/heads/master, refs/remotes/origin/master, refs/remotes/origin/HEAD)
Author: afrikha <email address hidden>
Date: Tue Aug 24 14:14:35 2021 -0400

    Add support for containerd v2 config (#50)

    * add support for containerd config v2

    * remove note from README

    * Update README.md

    * explicitly state the value v2 to set it to the new format

    Co-authored-by: canonical-deployer <email address hidden>

commit 475f8e3c4f3f7f1893aecc443f420901cf195984 (refs/remotes/origin/stable)
Author: Cory Johns <email address hidden>
Date: Fri Jun 18 14:48:54 2021 -0400

    Fix unit tests to work with new unitdata support (#49)

    The charms.unit_test library was updated to include better support for
    unitdata, removing the need to treat unitdata as a mock.
```

[Bartosz Woronicz (mastier1)]

Created fix for that:
https://github.com/charmed-kubernetes/charm-containerd/pull/52