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/need.sh

@@ -0,0 +1,9 @@
+#!/bin/sh
+fail=0
+for d in "$@"; do
+	if ! type "$d" >/dev/null 2>/dev/null; then
+		echo " -- missing tool: $d" >&2
+		fail=1
+	fi
+done
+exit "$fail"