Add a second docker registry as backup

[thomaspeitz]

Hey :)

Would it be possible to push the image not only to dockerhub but maybe to gcr / ecr / quay as well?

We would like to be less dependent on a particular docker registry (in cases of troubles)
and currently verify where alternative images are laying. We did not find any source for etcd-manager.

Greetings, Thomas

[thomaspeitz]

Another nice option would be that the docker image gets pushed as github release artifact like https://github.com/flannel-io/flannel/releases - Which would allow us to quickly upload the image to our own registry without taking care of a working build process in cases of troubles.