support forwarding signals

[AkihiroSuda]

Before:

$ dockerd-rootless.sh --experimental
...
INFO[2020-03-15T11:02:16.526051714+09:00] API listen on /run/user/1001/docker.sock     
^C
$ echo $?
130

After:

$ dockerd-rootless.sh --experimental
...
INFO[2020-03-15T11:02:42.263959577+09:00] API listen on /run/user/1001/docker.sock     
^CINFO[2020-03-15T11:02:43.493624366+09:00] Processing signal 'interrupt'                
INFO[2020-03-15T11:02:43.494655209+09:00] Processing signal 'interrupt'                
INFO[2020-03-15T11:02:43.495386457+09:00] stopping event stream following graceful shutdown  error="<nil>" module=libcontainerd namespace=moby
INFO[2020-03-15T11:02:43.495546067+09:00] Daemon shutdown complete                     
INFO[2020-03-15T11:02:43.495566660+09:00] stopping event stream following graceful shutdown  error="context canceled" module=libcontainerd namespace=plugins.moby
INFO[2020-03-15T11:02:43.495587614+09:00] stopping healthcheck following graceful shutdown  module=libcontainerd
INFO[2020-03-15T11:02:43.495986508+09:00] Processing signal 'interrupt'
$ echo $?
0

The implementation is based on /~https://github.com/docker/cli/blob/ef2f64abbd37edfa148f745fa0013731b5074d1b/cli/command/container/tty.go#L99-L12

[AkihiroSuda]

cc @ibuildthecloud

[tianon]

Is there a reason this isn't just doing signal.Notify(sigc)? (which should catch all signals without an explicit list)

(Also opencontainers/runc#2257, which I'm guessing you've probably already seen 😄)

[AkihiroSuda]

copypasta from moby 😓