fix: padded base64 encoded docker auth field

base64 allows usage of new line characters and some tools use them.
As a result, the length of the encoded string cannot be used to
determine whether it's padded or not.

This patch fixes the regression after #82148.

pkg/credentialprovider/config.go

@@ -287,8 +287,7 @@ func decodeDockerConfigFieldAuth(field string) (username, password string, err e
 
 	// StdEncoding can only decode padded string
 	// RawStdEncoding can only decode unpadded string
-	// a string is correctly padded if and only if its length is a multiple of 4
-	if (len(field) % 4) == 0 {
+	if strings.HasSuffix(strings.TrimSpace(field), "=") {
 		// decode padded data
 		decoded, err = base64.StdEncoding.DecodeString(field)
 	} else {