Killing a Docker container running hackage-server causes it to fail to start again

I’m not currently sure if this is an issue with hackage-server, acid-state, Docker, or something else, but I figured I would report it all the same. Here are the steps I’ve used to reproduce the issue:

```
$ docker run -d --name hackage-server -v hackage-state:/runtime/state hackage-server
$ docker kill hackage-server
$ docker run -it -v hackage-state:/runtime/state hackage-server
hackage-server: Guessing public URI as http://f5143d805ae8:8080
(you can override with the --base-uri= flag)
state/db/Users/open.lock: Locked by 5: resource busy
```

I’m not sure what is causing that error, but it seems to be the opposite of what I would expect: my understanding was that the _whole point_ of acid-state is to be able to recover from ungraceful shutdown.