root dir for execdriver (/var/run/docker) should be configurable

Signed-off-by: Shishir Mahajan <shishir.mahajan@redhat.com>

contrib/completion/bash/docker

@@ -1161,6 +1161,7 @@ _docker() {
 		--dns-search
 		--exec-driver -e
 		--exec-opt
+		--exec-root
 		--fixed-cidr
 		--fixed-cidr-v6
 		--graph -g

docs/sources/reference/commandline/cli.md

@@ -152,6 +152,7 @@ expect an integer, and they can only be specified once.
       --default-ulimit=[]                    Set default ulimit settings for containers
       -e, --exec-driver="native"             Exec driver to use
       --exec-opt=[]                          Set exec driver options
+      --exec-root="/var/run/docker"          Root of the Docker execdriver
       --fixed-cidr=""                        IPv4 subnet for fixed IPs
       --fixed-cidr-v6=""                     IPv6 subnet for fixed IPs
       -G, --group="docker"                   Group for the unix socket