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/dockjson.py

@@ -0,0 +1,21 @@
+#!/usr/bin/env python
+"""Generate a docker 1.0-style manifest for a docker image."""
+import json, os, subprocess, sys, time
+
+j = json.load(open('template.json'))
+layerid = open(sys.argv[1] + '.list.sha256').read().strip()
+j['id'] = layerid
+
+if len(sys.argv) > 2 and sys.argv[2]:
+    parentid = open(sys.argv[2] + '.list.sha256').read().strip()
+    j['parent'] = parentid
+
+t = time.time()
+gt = time.gmtime(t)
+nsec = int(t * 1e9) % 1000000000
+j['created'] = time.strftime('%Y-%m-%dT%H:%M:%S', gt) + ('.%09dZ' % nsec)
+
+nbytes = os.stat(sys.argv[1] + '.layer').st_size
+j['Size'] = nbytes
+
+json.dump(j, sys.stdout, indent=2)