Implement detached runs for ECS backend.

[ejholmes]

Closes #570

This implements detached runs for ECS.

This is a continuation of #573, with the issues presented in #573 (comment) addressed so that detached processes show up in emp ps:

$ emp run sleep 60 -d -a acme-inc
Ran `sleep 60` on acme-inc as run, detached.
$ emp ps -a acme-inc
v1.run.eddbb21a-1b89-4f0e-bb72-8193c54e1cb7  1X  RUNNING   3s  "sleep 60"
v1.web.907a716b-84d0-46f4-affe-616c2dc10399  1X  RUNNING   2h  "acme-inc server"

[ejholmes]

This ended up being really simple. Just use the app id here, which makes it easy to filter for tasks started by the app in ListTasks.

[phobologic]

👍

[crohr]

Awesome! What happens to the logs? Can we somehow get the exit code of the command?

[ejholmes]

@crohr the logs will end up in the same location as the logs for normal processes of apps, which is generally just on disc (we use logspout to shuttle these off to sumo logic).

Regarding the exit code, you can take the UUID from emp ps and use the aws ecs describe-tasks command to get the exit code. It'd be nice to eventually have Empire expose that information though.