restrict bash completion for hostdir arg to directories

The previous state assumed that the HOSTPATH argument referred to a
file. As clarified by moxiegirl in PR #11305, it is a directory.
Adjusted completion to reflect this.

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

contrib/completion/bash/docker

@@ -325,7 +325,7 @@ _docker_cp() {
 			(( counter++ ))
 
 			if [ $cword -eq $counter ]; then
-				_filedir
+				_filedir -d
 				return
 			fi
 			;;