Improve "invalid reference" error message

Use the same error message as is used in
docker/reference/reference.go to provide
slightly more information about the error.

This change tests if the reference passes
after lowercasing its characters, to determine
if the reference was invalid due to
it containing uppercase characters.

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

reference/reference_test.go

@@ -95,6 +95,25 @@ func TestReferenceParse(t *testing.T) {
 			input: "validname@invaliddigest:ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff",
 			err:   digest.ErrDigestUnsupported,
 		},
+		{
+			input: "Uppercase:tag",
+			err:   ErrNameContainsUppercase,
+		},
+		// FIXME "Uppercase" is incorrectly handled as a domain-name here, therefore passes.
+		// See https://github.com/docker/distribution/pull/1778, and https://github.com/docker/docker/pull/20175
+		//{
+		//	input: "Uppercase/lowercase:tag",
+		//	err:   ErrNameContainsUppercase,
+		//},
+		{
+			input: "test:5000/Uppercase/lowercase:tag",
+			err:   ErrNameContainsUppercase,
+		},
+		{
+			input:      "lowercase:Uppercase",
+			repository: "lowercase",
+			tag:        "Uppercase",
+		},
 		{
 			input: strings.Repeat("a/", 128) + "a:tag",
 			err:   ErrNameTooLong,