stop printer after containers where stopped by SIGTERM

[ndeloof]

When compose is ran attached by compose up, and user interrupt by Ctrl+C, the log printer is not notified about container termination and compose is stuck waiting.

This PR run watchContainers with a background context so it won't get canceled by the top level signal management (see cmd/compose.go#AdaptCmd). Doing so, the logPrinter will recieve ContainerExit events after stop sequence has started, and detect application is down to terminate with adequate exitcode

How to test:

1. Run compose app with docker compose up

services:
  web:
    image: nginx

2. Hit Ctrl+C
3. See notification for container being "Stopped", but still compose command doesn't exit