Docker Images on Dockerhub not up to date

The [release tab](https://github.com/cloudflare/complainer/releases) shows released versions of v1.6.1 already.
The [docker hub site of complainer](https://hub.docker.com/r/cloudflare/complainer/tags/) shows the latest 1.4 version tagged.

The `latest` tag "seems" to be up to date with the master. But only the time ("6 month ago") is the matching point here.

Proposal:
- Build, tag and upload a docker image in every github release
- Document how to release a new version
- Script / Automate the release (maybe under the usage of [goreleaser](https://github.com/goreleaser/goreleaser)