support docker secrets for registry password retrieval

[rbauduin]

Signed-off-by: BAUDUIN Raphaël [email protected]

[djmaze]

Nice change, thanks!

I think if we allow supplying the password through secrets then we should do the same for the username. @rbauduin Would you be willing to implement that as well?

[rbauduin]

I wondered if the usename should also be in a secret, but it seems the docker themselves only bother about the password.
From my understanding hiding usernames is usually related to protecting privacy, phishing attacks of brute force attacks, which don't seem to apply here.
If you think it still has an added value, I would propose to do it in separate pull request

[djmaze]

Sorry for leaving this lying around for so long. It's good to merge and we can open a separate PR if we want to allow this for the username as well.