Specifying the proc with run-app-docker-opts

It's plausible that an app could want Docker opts to only apply for the web proc container (or another named proc).
- We could specify it as an environment variable.
- We could specify it as another argument to run-app-docker-opts.
- We could have separate hooks for run-app-web-docker-opts and the rest.

Of the three, I'm leaning toward the first or second one.
