container.go: support mapped-volumes-from to map volumes by name
container.go: support mapped-volumes-from to map volumes by name

using volumes-from imports all volumes from another container, but gives
you no choice about the path the volume is imported at.  You can
only choose names when mapping a volume from the host.  This commit
allows you to import volumes from another container at a named path.

in particular we want our nginx container to be able to import the
same path from multiple app instances to unique names in the nginx
container.

ScientificRevenue/crane