Fix docker-credential-osxkeychain list behaviour in case of missing entry in keychain

[pgayvallet]

Currently, docker-credential-osxkeychain list returns an error if the keychain entry is missing.

However, docker logout actually clears the Docker Credentials keychain entry. Meaning that after performing the logout, the calls to docker-credential-osxkeychain list returns an error.

When building images (at least with docker-compose build, did not try directly with docker build) calls to docker-credential-osxkeychain list are performed, resulting in build failure as the credential helpers returns a non-0 return code.

This is probably not the expected behaviour. Also, this is not the case on windows, as the used implementation from danieljoos/wincred correctly handles the case:

func List() ([]*Credential, error) {
	creds, err := nativeCredEnumerate("", true)
	if err != nil && err.Error() == naERROR_NOT_FOUND {
		// Ignore ERROR_NOT_FOUND and return an empty list instead
		creds = []*Credential{}
		err = nil
	}
	return creds, err
}

This PR changes the docker-credential-osxkeychain list behaviour to return an empty map if the keychain entry is missing.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "osx-list-no-error-on-missing-key" git@github.com:pgayvallet/docker-credential-helpers.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[silvin-lubecki]

Seems legit 😸
Could you add a comment in the code with a short explanation ? (just a summary of the one you made in the PR)
Also is there any simple way to make this behavior explicit now in a unit test?

[gtardif]

Also checked the resulting binary on mac, docker-compose up --build works fine now