Improved Docker image for Grafana

[xlson]

Important: This won't be merged until we are ready to release Grafana 5.1.

There are a number of problems with the current setup that limits its usefulness. There is a high likelyhood that some of these changes will break backwards compatibility.

This issue tracks the different issues with the current docker image and where we are in the implementation.

• remove chown when starting
  • usage of chown makes it impossible to map in read-only volumes (Can't start Grafana on Kubernetes 1.7.14, 1.8.9, or 1.9.4 #140, Bind-mounting read-only grafana.ini causes: chown: changing ownership of '/etc/grafana/grafana.ini': Read-only file system #51)
• limit the number of volumes used in the docker container to make it easier to override config files and such (Remove VOLUME declaration from Dockerfile (or propose method for changing config on a remote docker host) #50)
• make it so that it's possible to configure what user grafana runs as
  • this is important when using volume mappings from the local host (Use nobody user in etc #124, Allow custom user to be specified #102 )
• Update docs
  • changes needed due to changes in the container
  • handling volume mappings and host binds and runnings Grafana as a different user
  • upgrading from the old container image to the new
• Test all use cases listed in the Readme
• Decide how to handle breaking changes
  • the container no longer runs as the same userid by default which will be a problem when upgrading
  • should we update existing containers?

In progress here: https://github.com/grafana/grafana-docker/tree/image-improvements