allow use of host:port in a FROM instruction

when having a FROM line like: `FROM myregistry:port/imagename:tag`, there is an error `unexpected ':' expecting '@', a new line followed by the next instruction, at least one space, or the image tag`.
This patch proposes to simplify how the registry is computed.
fixes: hadolint/hadolint#355

I don't know haskell, I tested the change by cloning the repo, and testing on a local file using the test in integration-tests

src/Language/Docker/Parser/From.hs

@@ -9,11 +9,9 @@ import Language.Docker.Syntax
 
 parseRegistry :: (?esc :: Char) => Parser Registry
 parseRegistry = do
-  domain <- someUnless "a domain name" (== '.')
-  void $ char '.'
-  tld <- someUnless "a TLD" (== '/')
+  registry <- someUnless "a registry" (== '/')
   void $ char '/'
-  return $ Registry (domain <> "." <> tld)
+  return $ Registry (registry)
 
 parsePlatform :: (?esc :: Char) => Parser Platform
 parsePlatform = do