fix(cmd/docker): prevent race between force-exit goroutine and plugin wait

When a plugin ignores context cancellation and the user sends 3 SIGINTs,
the CLI kills the plugin with SIGKILL. Previously the signal goroutine
called os.Exit(1) directly; a race existed where plugincmd.Run() could
return first (plugin was SIGKILL'd, so ws.ExitStatus() = -1) and the
main goroutine would call os.Exit(-1) = exit code 255 before the
goroutine reached os.Exit(1).

Fix by moving exit-code ownership to the main goroutine. The signal
goroutine closes forceExitCh before calling Kill(), guaranteeing the
channel is closed before plugincmd.Run() returns (the plugin can only
die after Kill() delivers SIGKILL; Run() only returns after the process
is reaped). The main goroutine checks forceExitCh after Run() returns
and performs the print + os.Exit(1) itself.

Also return from the signal goroutine after the force-kill to prevent
further loop iterations from calling close(forceExitCh) a second time
(which would panic), in case additional signals arrive while the kill
is in flight.

Fixes a flaky failure in TestPluginSocketCommunication/detached/
the_main_CLI_exits_after_3_signals where exit code 255 was observed
instead of 1 on loaded CI runners (RC Docker on Alpine).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Michael Zampani <michael.zampani@docker.com>