Merge pull request #6922 from tianon/bash-complete-build-directories

Update bash completion to only complete on directories as the argument to "docker build"

contrib/completion/bash/docker

@@ -156,7 +156,7 @@ _docker_build()
 		*)
 			local counter="$(__docker_pos_first_nonflag '-t|--tag')"
 			if [ $cword -eq $counter ]; then
-				_filedir
+				_filedir -d
 			fi
 			;;
 	esac