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.list.do

@@ -0,0 +1,24 @@
+d=$PWD
+redo-ifchange "$2.fs"
+
+if [ -e "$2.diffbase" ]; then
+	redo-ifchange "$2.diffbase"
+	read diffbase <$2.diffbase
+	diffbase=$diffbase.list
+	redo-ifchange "$diffbase"
+else
+	diffbase=/dev/null
+	redo-ifcreate "$2.diffbase"
+fi
+
+(
+	cd "$2" &&
+	find . -print | sort | "$d/try_fakeroot.sh" "$d/$2.fakeroot" "$d/fileids.py"
+) >$1.tmp
+
+comm -1 -3 "$diffbase" "$1.tmp" >$3
+rm -f "$1.tmp"
+
+# Sanity check
+nbytes=$(wc -c <"$3")
+test $nbytes -gt 0