`stack exec` not propagating signals when using Docker

[nh2]

This is exactly the same issue as #527, but when you have a stack.yaml that is Docker-enabled:

docker:
  enable: true

I falsely assumed that #527 was also about the Docker case, so please refer to #527 (comment) for a description of the problem.

This is the Docker issue that I believe would resolve this problem automatically if implemented: moby/moby#7567

[borsboom]

Not holding my breath on the upstream Docker issue getting fixed. Solution is probably to have stack catch signals and then proxy those itself using docker kill -s <signal>, at least for sigTERM and maybe some others.

[borsboom]

I've improved this significantly, and it at least works for sigTERM now. The container's current PID 1 (phusion/baseimage's my_init) doesn't pass other signals along to subprocesses, so I also have a workaround that, in non-interactive contexts, proxies a sigINT received by stack to a sigTERM for the container (this makes stopping a Bamboo/Jenkins task work properly, but keeps Ctrl-C working normally for other cases).

To fully fix this, we'd need a different PID 1. One approach would be to have stack itself act as PID 1, and have it pass along signals to its subprocesses, as well as reap zombie processes.

@nh2 Can you try the current master out for your use case?

[borsboom]

Blocked by #531.