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/container/memcalc.py

@@ -0,0 +1,8 @@
+#!/usr/bin/env python
+import os, sys
+st = os.stat(sys.argv[1])
+megabytes = st.st_size // 1024 // 1024
+# initrd size is limited to 50% of available RAM.  To be safe, we'll
+# request at least 3x initrd size, and no less than 64M.
+need = megabytes * 3 + 64
+print("%dM" % need)