Added Dockerfile suitable for DockerHub Auto-Builds

[dcwangmit01]

Hi @fstab,

This PR adds a Dockerfile for grok_exporter which builds an alpine linux docker image which contains a statically compiled grok_exporter binary. This should be suitable for DockerHub Auto-builds. If you wanted, you could go to dockerhub and create a connected public repository with the following build settings which will auto-build a "latest" docker image upon master branch changes, and auto-build a "vX.Y.Z" docker image upon repo tagging.

Thanks! -dave