Revert "Use RUN in readme example"

This reverts commit 539f1acdb8307d18e591442927b978043f2786e0.

README.md

@@ -79,7 +79,7 @@ web:
 
     FROM orchardup/rails
     ADD . /code
-    RUN bundle exec rackup
+    CMD: bundle exec rackup
 
 
 ### Communicating between containers