New command gameserver log

## User Story

As a volunteer trying to help and improve the Docker container implementation, I want a new command like **gameserver log** to output the gameserver log into stdout like **gameserver debug** does.

## Basic info

* **Distro:** general
* **Game:** general
* **Command:** log
* **LinuxGSM version:** [v20.1.3]

## Further Information

I use a run.sh as entrypoint and therefore i need a proper PID 1 process for the container.
Due to the fact, that _gameserver start_ spawns the server as new process, it can't be used.
Therefor it would be perfect, if some **gameserver log** command would exist, that outputs continuously the gameserver logs into stdout like _gameserver debug_ does and runs as long as the gameserver.

Then, the run.sh called as entrypoint could look like this:
`#!/bin/bash`
`....`
`su -c "gameserver start" $user`
`su -c "gameserver log" $user` <------ running as long as the gameserver runs

With this approach, the _docker logs_ command would flawlessly provide all the logging from the gameserver and the container would be stopped if the gameserver is.
And maybe a **gameserver log** command could provide handy information in other use cases than in a container environment.

Cheers