Improve docker lexer

* Allow comments between parts of multiline run instructions
* And systax highlighting for label keys and values

lib/rouge/lexers/docker.rb

@@ -12,7 +12,7 @@ class Docker < RegexLexer
       mimetypes 'text/x-dockerfile-config'
 
       KEYWORDS = %w(
-        FROM MAINTAINER CMD LABEL EXPOSE ENV ADD COPY ENTRYPOINT VOLUME USER WORKDIR ARG STOPSIGNAL HEALTHCHECK SHELL
+        FROM MAINTAINER CMD EXPOSE ENV ADD COPY ENTRYPOINT VOLUME USER WORKDIR ARG STOPSIGNAL HEALTHCHECK SHELL
       ).join('|')
 
       start { @shell = Shell.new(@options) }
@@ -40,16 +40,37 @@ class Docker < RegexLexer
           @shell.reset!
         end
 
+        rule %r/^LABEL(\s+)/i do
+          token Keyword
+          push :label
+        end
+
         rule %r/\w+/, Text
         rule %r/[^\w]+/, Text
         rule %r/./, Text
       end
 
       state :run do
         rule %r/\n/, Text, :pop!
+        rule %r/^\s*#.*\n/, Comment
         rule %r/\\./m, Str::Escape
         rule(/(\\.|[^\n\\])+/) { delegate @shell }
       end
+
+      state :label do
+        rule %r/\n/, Text, :pop!
+        rule %r/^\s*#.*\n/, Comment
+        rule %r/\s*\\./m, Str::Escape
+        rule %r/(\s*(?:\S+|"[^"]+"))(=)/ do
+          groups Name::Property, Punctuation
+          push :value
+        end
+      end
+
+      state :value do
+        rule %r/\n/, Text, :pop!
+        rule %r/(".*?")|('.*?')/m, Str, :pop!
+      end
     end
   end
 end