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/try_fakeroot.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+frfile=$1
+shift
+broken=
+fakeroot true 2>/dev/null || broken=1
+if [ -z "$broken" ] && [ -e "$frfile" ]; then
+	redo-ifchange "$frfile"
+	exec fakeroot -i "$frfile" "$@"
+elif [ -z "$broken" ]; then
+	exec fakeroot "$@"
+else
+	exec "$@"
+fi