Use net.IP.IsLoopback() to match loopback addresses

- changed the `MatchLocalhost` function in remotes/docker/registry.go
	- Make use of SplitHostPort to split host and port number
- Added additional tests for modified code in remotes/docker/registry_test.go
- Note: this does not handle mathcing of IP addresses in octal, decimal or hex format or a mix of these.

Fixes: #5129

Signed-off-by: Madhav Jivrajani <madhav.jiv@gmail.com>

remotes/docker/registry_test.go

@@ -52,14 +52,19 @@ func TestMatchLocalhost(t *testing.T) {
 		match bool
 	}{
 		{"", false},
-		{"127.1.1.1", false},
+		{"127.1.1.1", true},
 		{"127.0.0.1", true},
+		{"127.256.0.1", false}, // test MatchLocalhost does not panic on invalid ip
+		{"127.23.34.52", true},
 		{"127.0.0.1:5000", true},
 		{"registry.org", false},
+		{"126.example.com", false},
 		{"localhost", true},
 		{"localhost:5000", true},
 		{"[127:0:0:1]", false},
 		{"[::1]", true},
+		{"[::1]:", false},     // invalid ip
+		{"127.0.1.1:", false}, // invalid ip
 		{"[::1]:5000", true},
 		{"::1", true},
 	} {