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/default.sha256.do

@@ -0,0 +1,16 @@
+#!/usr/bin/env python
+"""Calculate the sha256 digest of a given file."""
+import hashlib, os, subprocess, sys
+
+subprocess.check_call([
+    'redo-ifchange',
+    sys.argv[2],
+])
+
+h = hashlib.sha256()
+f = open(sys.argv[2])
+while 1:
+    b = f.read(65536)
+    if not b: break
+    h.update(b)
+open(sys.argv[3], 'w').write(h.hexdigest() + '\n')