builder: Fix handling of VOLUME command where multiple volumes are
specified in a space delimited list.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Merge pull request #7994 from erikh/parser_fix_volume_parsing

builder: Fix handling of VOLUME command where multiple volumes are specified in a space delimited list.