https://docs.cloudfoundry.org/buildpacks/custom.html
mkdir -m 0755 /nix && chown vcap /nix
su vcap
. /home/vcap/.nix-profile/etc/profile.d/nix.sh
nix-env --install darkhttpd
nix-env --install cowsay
cowsay "hello from staging!!!"
darkhttpd `pwd` --daemon --port $PORT