cli/config/credentials: skip saving config-file if credentials didn't change

Before this change, the config-file was always updated, even if there
were no changes to save. This could cause issues when the config-file
already had credentials set and was read-only for the current user.

For example, on NixOS, this poses a problem because `config.json` is a
symlink to a write-protected file;

    $ readlink ~/.docker/config.json
    /home/username/.config/sops-nix/secrets/ghcr_auth

    $ readlink -f ~/.docker/config.json
    /run/user/1000/secrets.d/28/ghcr_auth

Which causes `docker login` to fail, even if no changes were to be made;

    Error saving credentials: rename /home/derek/.docker/config.json2180380217 /home/username/.config/sops-nix/secrets/ghcr_auth: invalid cross-device link

This patch updates the code to only update the config file if changes
were detected. It there's nothing to save, it skips updating the file,
as well as skips printing the warning about credentials being stored
insecurely.

With this patch applied:

    $ docker login -u yourname
    Password:

    WARNING! Your credentials are stored unencrypted in '/root/.docker/config.json'.
    Configure a credential helper to remove this warning. See
    https://docs.docker.com/go/credential-store/

    Login Succeeded

    $ docker login -u yourname
    Password:
    Login Succeeded

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah

cli/config/credentials/file_store.go

@@ -30,6 +30,10 @@ func NewFileStore(file store) Store {
 
 // Erase removes the given credentials from the file store.
 func (c *fileStore) Erase(serverAddress string) error {
+	if _, exists := c.file.GetAuthConfigs()[serverAddress]; !exists {
+		// nothing to do; no credentials found for the given serverAddress
+		return nil
+	}
 	delete(c.file.GetAuthConfigs(), serverAddress)
 	return c.file.Save()
 }
@@ -70,9 +74,14 @@ https://docs.docker.com/go/credential-store/
 // CLI invocation (no need to warn the user multiple times per command).
 var alreadyPrinted atomic.Bool
 
-// Store saves the given credentials in the file store.
+// Store saves the given credentials in the file store. This function is
+// idempotent and does not update the file if credentials did not change.
 func (c *fileStore) Store(authConfig types.AuthConfig) error {
 	authConfigs := c.file.GetAuthConfigs()
+	if oldAuthConfig, ok := authConfigs[authConfig.ServerAddress]; ok && oldAuthConfig == authConfig {
+		// Credentials didn't change, so skip updating the configuration file.
+		return nil
+	}
 	authConfigs[authConfig.ServerAddress] = authConfig
 	if err := c.file.Save(); err != nil {
 		return err