Add zsh completion for 'dockerd --init' and 'docker {create,run} --init'

Signed-off-by: Steve Durrheimer <s.durrheimer@gmail.com>

contrib/completion/zsh/_docker

@@ -541,6 +541,7 @@ __docker_container_subcommand() {
         "($help)*--group=[Set one or more supplementary user groups for the container]:group:_groups"
         "($help -h --hostname)"{-h=,--hostname=}"[Container host name]:hostname:_hosts"
         "($help -i --interactive)"{-i,--interactive}"[Keep stdin open even if not attached]"
+        "($help)--init[Run an init inside the container that forwards signals and reaps processes]" \
         "($help)--ip=[IPv4 address]:IPv4: "
         "($help)--ip6=[IPv6 address]:IPv6: "
         "($help)--ipc=[IPC namespace to use]:IPC namespace: "
@@ -2370,6 +2371,7 @@ __docker_subcommand() {
                 "($help -g --graph)"{-g=,--graph=}"[Root of the Docker runtime]:path:_directories" \
                 "($help -H --host)"{-H=,--host=}"[tcp://host:port to bind/connect to]:host: " \
                 "($help)--icc[Enable inter-container communication]" \
+                "($help)--init[Run an init inside containers to forward signals and reap processes]" \
                 "($help)--init-path=[Path to the docker-init binary]:docker-init binary:_files" \
                 "($help)*--insecure-registry=[Enable insecure registry communication]:registry: " \
                 "($help)--ip=[Default IP when binding container ports]" \