This is a smaller (25 MB) etcd image built from the progrium/busybox base. We needed an image that contained /bin/echo so that it was compatibile with Fig. All other etcd images found were based on ubuntu or something larger.
There are two configuration environment variables set, ETCD_DATA_DIR which is a Docker volume at /etcddata, and ETCD_NAME which is set to etcd. See https://coreos.com/docs/distributed-configuration/etcd-configuration/ for environment variables that can be passed for configuration.