Make zsh completion work when ps output is custom

When `psFormat` is used in the docker client config json, if the output
is non-standard it breaks some of the completion handling for
containers.

This fixes that by ensuring that calls to `ps` use the default/standard
formatting by calling `docker ps --format 'table'`

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

contrib/completion/zsh/_docker

@@ -57,7 +57,7 @@ __docker_get_containers() {
     type=$1; shift
     [[ $kind = (stopped|all) ]] && args=($args -a)
 
-    lines=(${(f)"$(_call_program commands docker $docker_options ps --no-trunc $args)"})
+    lines=(${(f)"$(_call_program commands docker $docker_options ps --format 'table' --no-trunc $args)"})
 
     # Parse header line to find columns
     local i=1 j=1 k header=${lines[1]}