cookbook/container/default.sha256.do: use explicit close_fds=False.

Some people have /usr/bin/python as a link to python3. The script is
designed to work in either python2 or python3, but python3's subprocess
model defaults to close_fds=True, which closes the jobserver fds and
causes an error.  Explicitly force close_fds=False to get identical
behaviour on python2 and python3.

Reported-by: Tharre <tharre3@gmail.com>

docs/cookbook/container/dockjson.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python
 """Generate a docker 1.0-style manifest for a docker image."""
-import json, os, subprocess, sys, time
+import json, os, sys, time
 
 j = json.load(open('template.json'))
 layerid = open(sys.argv[1] + '.list.sha256').read().strip()