Set up automatic docker builds and add docker pull to README

[audip]

As discussed in the PR comments, a feature request for this image to be publically available on Docker Hub that way one can docker run <author>/<container-image>:<tag> to pull and start using fluentd-exporter without having to build the image themselves.

The addition of the image to readme will be beneficial and only those who make changes to the code will need to build image for themselves.

[V3ckt0r]

Yea that's cool. I will take a look at this.

[V3ckt0r]

linking PR for docker CI #5

[V3ckt0r]

Successful CI builds now push to docker hub https://hub.docker.com/r/vect0r/fluentd_exporter/

[audip]

Thank you. 🎉