Expose DomainRegexp from reference

This fix is based on:
https://github.com/docker/docker/pull/30746#discussion_r99650885

The goal is to reuse the `DomainRegexp` in docker to check
for `<host>:<port>` pattern.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

reference/regexp_test.go

@@ -116,7 +116,7 @@ func TestDomainRegexp(t *testing.T) {
 			match: true,
 		},
 	}
-	r := regexp.MustCompile(`^` + domainRegexp.String() + `$`)
+	r := regexp.MustCompile(`^` + DomainRegexp.String() + `$`)
 	for i := range hostcases {
 		checkRegexp(t, r, hostcases[i])
 	}