Updated Dockerfile
Updated Dockerfile

- Added libffi-dev for PyNaCl, Fixes SexualRhinoceros/MusicBot#171
- Installation of PIP-dependencies does now happen during build

Crims101/SymmetraBot,reddit-diabetes/musicbot-cloud,GeoffreyWesthoff/DJWillex,Crims101/SymmetraBot,reddit-diabetes/musicbot-cloud,GeoffreyWesthoff/DJWillex,DiscordMusicBot/MusicBot