Allow String? for 'docker' RuntimeAttribute

In other words, we should validate docker as an optional string so that we can do things like this:

task foo {
  String? dockerName
  command { ... }
  runtime {
    docker: dockerName
  }
  output { ... }
}

The expected behaviour here is:

• If dockerName is set, use docker and use the specified image name
• If dockerName is not set, do not use docker.

[LeeTL1220]

@ruchim We are getting a lot of questions about being able to run local/SGE without docker at the Spain workshop… Not just Spanish people, but from several countries.

[aednichols]

Another point that Eric and I came across is that a “Docker in Docker” solution - i.e. installing Docker inside the Docker container where he’s running Cromwell - is not good either because it necessitates pushing and re-pulling the Docker image he’s iterating on, which makes for annoyingly long cycle times and can’t work with bad or no Internet.