Fix problem with = in completion of docker global options

Without this fix, `docker --log-driver fluentd --log-opt fluentd-tag=b`
would complete `b` to `build`.
Completion of the commands has to be nailed to __docker_pos_first_nonflag

Signed-off-by: Harald Albers <github@albersweb.de>

contrib/completion/bash/docker

@@ -359,7 +359,10 @@ _docker_docker() {
 			COMPREPLY=( $( compgen -W "$boolean_options $main_options_with_args" -- "$cur" ) )
 			;;
 		*)
-			COMPREPLY=( $( compgen -W "${commands[*]} help" -- "$cur" ) )
+			local counter="$(__docker_pos_first_nonflag $main_options_with_args_glob)"
+			if [ $cword -eq $counter ]; then
+				COMPREPLY=( $( compgen -W "${commands[*]} help" -- "$cur" ) )
+			fi
 			;;
 	esac
 }