Documentation: docker pull latest etcd from quay

As pointed out at https://github.com/coreos/etcd/issues/3843, Docker guide uses
outdated version of etcd. This docker commands will pull from the latest
releases.

Documentation/docker_guide.md

@@ -12,9 +12,11 @@ export HostIP="192.168.12.50"
 
 The following `docker run` command will expose the etcd client API over ports 4001 and 2379, and expose the peer port over 2380.
 
+This will run the latest release version of etcd. You can specify version if needed (e.g. `quay.io/coreos/etcd:v2.2.0`).
+
 ```
 docker run -d -v /usr/share/ca-certificates/:/etc/ssl/certs -p 4001:4001 -p 2380:2380 -p 2379:2379 \
- --name etcd quay.io/coreos/etcd:v2.0.8 \
+ --name etcd quay.io/coreos/etcd \
  -name etcd0 \
  -advertise-client-urls http://${HostIP}:2379,http://${HostIP}:4001 \
  -listen-client-urls http://0.0.0.0:2379,http://0.0.0.0:4001 \
@@ -44,7 +46,7 @@ The main difference being the value used for the `-initial-cluster` flag, which
 
 ```
 docker run -d -v /usr/share/ca-certificates/:/etc/ssl/certs -p 4001:4001 -p 2380:2380 -p 2379:2379 \
- --name etcd quay.io/coreos/etcd:v2.0.8 \
+ --name etcd quay.io/coreos/etcd \
  -name etcd0 \
  -advertise-client-urls http://192.168.12.50:2379,http://192.168.12.50:4001 \
  -listen-client-urls http://0.0.0.0:2379,http://0.0.0.0:4001 \
@@ -59,7 +61,7 @@ docker run -d -v /usr/share/ca-certificates/:/etc/ssl/certs -p 4001:4001 -p 2380
 
 ```
 docker run -d -v /usr/share/ca-certificates/:/etc/ssl/certs -p 4001:4001 -p 2380:2380 -p 2379:2379 \
- --name etcd quay.io/coreos/etcd:v2.0.8 \
+ --name etcd quay.io/coreos/etcd \
  -name etcd1 \
  -advertise-client-urls http://192.168.12.51:2379,http://192.168.12.51:4001 \
  -listen-client-urls http://0.0.0.0:2379,http://0.0.0.0:4001 \
@@ -74,7 +76,7 @@ docker run -d -v /usr/share/ca-certificates/:/etc/ssl/certs -p 4001:4001 -p 2380
 
 ```
 docker run -d -v /usr/share/ca-certificates/:/etc/ssl/certs -p 4001:4001 -p 2380:2380 -p 2379:2379 \
- --name etcd quay.io/coreos/etcd:v2.0.8 \
+ --name etcd quay.io/coreos/etcd \
  -name etcd2 \
  -advertise-client-urls http://192.168.12.52:2379,http://192.168.12.52:4001 \
  -listen-client-urls http://0.0.0.0:2379,http://0.0.0.0:4001 \