Fix notation of named stages in multi-stage docker builds (#1809)

* Fix notation of named stages in multi-stage docker builds

* Check for whitespace as well to mitigate "AS" appearing in image names

* Add multi-stage builder pattern to sample

lib/rouge/lexers/docker.rb

@@ -20,6 +20,10 @@ class Docker < RegexLexer
       state :root do
         rule %r/\s+/, Text
 
+        rule %r/^(FROM)(\s+)(.*)(\s+)(AS)(\s+)(.*)/io do
+          groups Keyword, Text::Whitespace, Str, Text::Whitespace, Keyword, Text::Whitespace, Str
+        end
+
         rule %r/^(ONBUILD)(\s+)(#{KEYWORDS})(.*)/io do
           groups Keyword, Text::Whitespace, Keyword, Str
         end

spec/visual/samples/docker

@@ -1,3 +1,4 @@
+FROM golang:1.16 AS builder
 maintainer First O'Last
 
 run echo \