SIGUSR1 not received by task

### Nomad version
Nomad v0.10.2

### Operating system and Environment details
Darwin 19.3.0 Darwin Kernel Version 19.3.0: Thu Jan 9 20:58:23 PST 2020; root:xnu-6153.81.5~1/RELEASE_X86_64 x86_64

Docker version 19.03.5, build 633a0ea

### Issue

Attempting to send a SIGUSR1 using `nomad alloc signal -s SIGUSR1` does not reach the task.
Other signals sent with `alloc signal` such as `SIGHUP` and `SIGINT` work.

Sending the signal via `docker kill --signal="SIGUSR1"` works.
Sending signal via `kill` from inside the container works.

Process is running as PID 1 and root.

The nomad UI indicates the task is being sent a signal.

The Nomad log output:

```
2020-02-05T14:38:31.748-0600 [DEBUG] http: request complete: method=PUT path=/v1/client/allocation/04e2d45a-c713-cc6b-4613-678c2741b3e3/signal?region=global duration=44.861877ms
```

Reproduce the failure with `nomad alloc signal -s SIGUSR1` and compare it with the working SIGHUP and SIGINT cases. Start at the `alloc signal` entry point and trace how the signal is delivered to the Docker task; done means SIGUSR1 reaches the task as it does through Docker or from inside the container.

Written by the indexing model from the issue text.