Add dockerfile highlighter

syntax/docker.krk

@@ -0,0 +1,37 @@
+from syntax import Highlighter, bind
+from syntax.bash import BashHighlighter
+
+@bind
+class DockerfileHighlighter(BashHighlighter):
+    name = "docker"
+    extensions = ('Dockerfile',)
+    spaces = True
+
+    keywords = [
+        'FROM','AS','MAINTAINER','RUN','CMD','COPY','EXPOSE','ADD',
+        'ENTRYPOINT','VOLUME','USER','WORKDIR','ONBUILD','LABEL','ARG',
+        'HEALTHCHECK','SHELL','STOPSIGNAL',
+    ]
+
+    def calculate(self):
+        if self.state < 1 and self.i == 0:
+            if self[0] == '#':
+                self.paintComment()
+            else if self.matchAndPaint('RUN', self.FLAG_KEYWORD, self.cKeywordQualifier):
+                let out = super().calculate()
+                if out == None and self[-1] == '\\':
+                    return 1
+                return out
+            else if self.findKeywords(self.keywords, self.FLAG_KEYWORD, self.cKeywordQualifier):
+                return 0
+            else:
+                self.skip(1)
+        else:
+            if self.state == 1:
+                self.set_state(0)
+            let out = super().calculate()
+            if out == None and self[-1] == '\\':
+                return 1
+            return out
+        return None
+