make application container logs be configurable as spel

currently [log file name are not configurable](https://shinyproxy.io/documentation/configuration/#local-file-back-end)

```
<specId>_<proxyId>_<startupTime>_stdout.log
<specId>_<proxyId>_<startupTime>_stderr.log
```

might be nice to allow user to customise the names. At least in my case I need the userName (and had to patch the tool)