Implement librespot credential caching

[ironcrafter54]

Currently if you restart the container the cached librespot credentials will not be stored, if implementation/documentation on how to properly make these cached credentials persistent that would be great. This is what the librespot readme says about this:
Usage
A sample program implementing a headless Spotify Connect receiver is provided. Once you've built librespot, run it using :

target/release/librespot --name DEVICENAME
The above is a minimal example. Here is a more fully fledged one:

target/release/librespot -n "Librespot" -b 320 -c ./cache --enable-volume-normalisation --initial-volume 75 --device-type avr
The above command will create a receiver named Librespot, with bitrate set to 320 kbps, initial volume at 75%, with volume normalisation enabled, and the device displayed in the app as an Audio/Video Receiver. A folder named cache will be created/used in the current directory, and be used to cache audio data and credentials.

A full list of runtime options is available here.

Please Note: When using the cache feature, an authentication blob is stored for your account in the cache directory. For security purposes, we recommend that you set directory permissions on the cache directory to 700.