Add instructions to get Dockerfile working

Instruction to get Dockerfile working :+1: 
The  first two steps (clone repo + create local settings) are required for both of the installation methods (with/without docker), that's they are mentioned [at the beginning of the Installation section](https://github.com/jakubroztocil/cloudtunes#installation). So it would be a bit confusing to repeat one of the pre-requisites under one of the variants again. 

The local settings template is linked from multiple places and should be self-explanatory, so preferably just linking to it should be enough.