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/fileids.py

@@ -0,0 +1,21 @@
+#!/usr/bin/env python
+import hashlib, os, stat, sys
+
+for name in sys.stdin:
+    name = name[:-1]  # skip terminating newline
+    st = os.lstat(name)
+    if stat.S_ISREG(st.st_mode):
+        h = hashlib.sha256()
+        f = open(name)
+        while 1:
+            b = f.read(65536)
+            if not b: break
+            h.update(b)
+        digest = h.hexdigest()
+    elif stat.S_ISLNK(st.st_mode):
+        digest = hashlib.sha256(os.readlink(name)).hexdigest()
+    else:
+        digest = '0'
+    print('%s %07o-%s-%s-%s' % (
+        name,
+        st.st_mode, st.st_uid, st.st_gid, digest))