maybe better fill all non-null values into authconfig without combining them?

something like

AuthConfig authConfig = new AuthConfig;
if (getRegistryUsername() != null) {
    authConfig.setRegistryUsername(getRegistryUsername());
}
if(getRegistryPassword() != nul) {
    authConfig.setRegistryPassword(getRegistryPassword());
}
...

Objects.nonNull() and even in one liner

Ok, but in effectiveAuthConfig() the return value of getAuthConfig() is checked for null. This will no longer work with your approach

Then it will be empty, but not null...

yes and because it is not null the empty config will be used for authetification instead of getting the auth config from the docker config file. So in effectiveAuthConfig() we need to check if the auth config returned by getAuthConfig() is empty. Which I think is ugly.

Maybe we should change the && in ||:
if (getRegistryUsername() != null || getRegistryPassword() != null || getRegistryEmail() != null || getRegistryUrl() != null)

@KostyaSha What di you think?

well, if email optional, then ok as is