docs: add note about docker hub private images

[piksel]

Add findings in #796 (comment) to docs.

[codecov]

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Merging #1132 (f81d0b1) into main (d9d6f79) will not change coverage.
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[dschmidt]

I can not confirm this. For me only docker.io in image name and config.json worked. This did not.

[piksel]

@dschmidt Could you post the --debug lines corresponding these:

DEBU[0004] Trying to load authentication credentials.    container=/wt-test2 image="index.docker.io/piksel/wt-test:latest"
DEBU[0004] Loaded auth credentials for user piksel, on registry index.docker.io/piksel/wt-test:latest, from file /config.json

?

And the contents of your config.json (with the actual credentials removed of course).

[dschmidt]

Of course, happy to :)

This works:

watchtower_1         | time="2021-11-16T20:05:48Z" level=debug msg="Trying to load authentication credentials." container=/examples_projectxyz_1 image="docker.io/userxyz/projectxyz:tagxyz"
watchtower_1         | time="2021-11-16T20:05:48Z" level=debug msg="Loaded auth credentials for user userxyz, on registry docker.io/userxyz/projectxyz:tagxyz, from file /config/config.json"

with

{
        "auths": {
                "https://docker.io": {
                        "auth": "...."
                }
        }
}

and

  projectxyz:
    image: docker.io/userxyz/projectxyz:tagxyz

Actually this seems to work as well, although I was pretty sure I had tested it and it didn't work ... anyhow this seems to work actually:

watchtower_1         | time="2021-11-16T20:14:27Z" level=debug msg="Trying to load authentication credentials." container=/examples_nodered_1 image="index.docker.io/userxyz/projectxyz:tagxyz"
watchtower_1         | time="2021-11-16T20:14:27Z" level=debug msg="Loaded auth credentials for user userxyz, on registry index.docker.io/userxyz/projectxyz:tagxyz, from file /config/config.json"

with

  projectxyz:
    image: index.docker.io/userxyz/projectxyz:tagxyz

{
        "auths": {
                "https://index.docker.io/v1/": {
                        "auth": "..."
                }
        }
}

which is what I get when simply logging in with docker login.

So false alarm, I think you are right. I'm just not sure this documentation actually helps, as this information is basically already available from https://github.com/containrrr/watchtower/blob/f81d0b17c19e6b885884ce2f94a3aab76e258e88/docs/private-registries.md#share-the-docker-configuration-file.
I for some reason just failed to apply it correctly, maybe because I somehow messed up reloading after making changes...

[piksel]

Do you mean the prefix in the docker compose for watchtower? I don't see anything that expressively tells you to add it to your other containers... right?