Some Dockerization changes

[dmuth]

As mentioned in #10, this is some work I did which makes the container be a little more generic, so that I can run hostyoself in relay mode in a container, and also tweaks COPY in the Dockerfile so that things like editing the Dockerfile itself don't trigger a full reubild.

And it's less a PR and more a "pull only if you want to" or "pick and choose individual things from this PR instead".

-- Doug