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.initrd.do

@@ -0,0 +1,9 @@
+redo-ifchange "$2.fs" rdinit
+d=$PWD
+fs=$2
+(
+	(cd "$fs" && find . -print0 |
+	 "$d/try_fakeroot.sh" "$d/$2.fakeroot" \
+	 	cpio -Hnewc -0 -o)
+	printf 'rdinit\0' | cpio -Hnewc -0 -o
+) >$3