docker auth: improve missing user/pwd

[Frankkkkk]

Specify the "missing username and password" error message. This makes debugging the action easier when for example mistyping the username or the password.

Thanks and cheers!

[crazy-max]

Thanks for this PR but I think the error message to users should remain generic. You can still check what value is empty in the login step (expand Run docker/login-action@v3):

[Frankkkkk]

Hi @crazy-max , thanks for your message.
Any reason why you want the error message to be generic? I genuinely can't see why?

These actions are also used outside Github (for example on Gitea or Forgejo. Basically on any act runner). On those, the details are not shown:

Thus why it adds more info
Thanks