There are no good multi-master solution with a HA focus available for Postgres, at least no open source ones.
The patroni project checks most of your other requirements: docker/containerized design with leader election/continuing operation via etcd or another external DCS. Not sure how easy upgrades are but I think the procedure is at least documented.
The patroni project checks most of your other requirements: docker/containerized design with leader election/continuing operation via etcd or another external DCS. Not sure how easy upgrades are but I think the procedure is at least documented.