Builtin: support docker deployment resource specifications

[rugwirobaker]

Is your feature request related to a problem? Please describe.

There is currently no way to specify resource limits for apps that use docker.

Describe the solution you'd like
Support resource limits in the docker plugin. An example deployment stanza would look like this:

deploy {
  use "docker" {
    service_port = "8080"
    cpu = "2"
    memory = "512"
  }
}

Describe alternatives you've considered
N/A

Explain any additional use-cases
It would very useful for the docker swarm community but I also believe there is value in it for people just coming off Heroku(which takes care of everything) with a single docker host who would want to prevent a rog container consuming all their host's resources.

Additional context
N/A