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.layer.do

@@ -0,0 +1,9 @@
+d=$PWD
+redo-ifchange "$2.fs" "$2.list"
+
+sed -e 's/ [^ ]*$//' <$2.list |
+(
+	cd "$2"
+	"$d/try_fakeroot.sh" "$d/$2.fakeroot" \
+		cpio -Hustar -o
+) >$3