Dockerfile: Reduce size and dependency, update etcd.
Dockerfile: Reduce size and dependency, update etcd.

The Dockerfile now only builds upon the publicly available Ubuntu docker image, thereby reducing the size of the image.
etcd release 2.0.12 was released today as well, updated that dependency.

zalando/patroni,zalando/patroni,jinty/patroni,sean-/patroni,jinty/patroni,pgexperts/patroni,pgexperts/patroni,sean-/patroni