[Feature] Improve documentation on Docker

## Description

First of all, the command in the readme doesn't work, it should use ` deezer/spleeter:3.8` because `latest` doesn't exist (I saw logged issue).

Secondly, I'm not very familiar with windows shells and there's no instructions. After carefully setting up paths I'm trying to run in Powershell

```
docker run -v $Env:SPLEETER_INPUT:/input -v $Env:SPLEETER_OUTPUT:/output -v $Env:SPLEETER_MODEL:/model -e MODEL_PATH=/model deezer/spleeter:3.8 separate -o /output /input/test.mp3
```

and I get

```
Error: Invalid value for 'FILES...': File '/input/test.mp3' does not exist.
```

I also tried using `${VAR_NAME}` and also surrounding each variant with quotes but this didn't seem to help.

so then I went to cmd.exe and did

```
run -v %SPLEETER_INPUT%:/input -v %SPLEETER_OUTPUT%:/output -v %SPLEETER_MODEL%:/model -e MODEL_PATH=/model deezer/spleeter:3.8 separate -o /output /input/test.mp3
docker: invalid reference format.
See 'docker run --help'.
```

but I get the same reference error.