Make Docker a first class citizen

[fabiang]

and allow Docker Hub to build the image from repository automatically.

I've change some things to make the project more useful when used together with Docker:

• The configuration file can now contain environment variables
  • Most setups now can easily be configured without the requirement of passing a custom config file
  • Note: I've used github.com/drone/envsubst because that package supports default values for environment variables
• The Dockerfile now is feature-complete; including git, a config with environment variables and the releases folder is now a volume
• Based on this changes it's now possible to automatically create official Docker images via Docker Hub

I've tested the creation of Docker images ob Docker Hub based on tags/branches in a private repository.

[Clivern]

Thanks @fabiang for your PR. Adding envsubst is really a good idea!
within the next days I will use envsubst with config.dist.yml too, adding build dir & logs dir as volume too and Update README so it is clear how to override with environment vars & create a new release.

[fabiang]

Let me know, when you need assitance with documentation

[Clivern]

Yeah, sure, thanks again @fabiang