Add a volume to persist `/go/pkg` between runs

[lawrencegripper]

I've been using a volume with go to persist the code that go mod pulls down between runs. It's sped things up after I restart a container.

Would you be happy for me to add this to the sample?

[Chuxel]

@lawrencegripper Apologies for the delay. Happy to take a PR on it, yes! You should be able to use new new mount property as well.