cookbook/container: example of building+running docker containers.

This got... long... and complicated.  But I think it's a really good
demonstration of getting redo to do complicated things elegantly.  At
least, I hope it is.

docs/cookbook/clean.do

@@ -1,3 +1,4 @@
+rm -f *~ .*~
 for d in */clean.do; do
     echo "${d%.do}"
 done | xargs redo