run: adjust the order in which elements are added to $PATH
run: adjust the order in which elements are added to $PATH

When building the slice of environment variables to add to the
configuration for a container that we're about to run, in case of
conflicts, we want the values from the base image or working container
to override the global defaults, and we want values that were passed to
us through the API to override them both.

In cases of conflicts, values which occur later in the slice override
values which occurred earlier, so we want to add them in this order:
* values from containers.conf
* values from the base image or working container
* values passed to us through the API

We previously applied the containers.conf defaults after the base image
or working container's value, and that meant that containers.conf's
values always took precedence over the values in the image.

Signed-off-by: Nalin Dahyabhai <f562fcb2a0b898bf0be1afc867ea92325d8f825f@redhat.com>

rhatdan/buildah,TomSweeneyRedHat/buildah,nalind/buildah-1,nalind/buildah,projectatomic/buildah,nalind/buildah,nalind/buildah-1,nalind/buildah-1,projectatomic/buildah,rhatdan/buildah,rhatdan/buildah,TomSweeneyRedHat/buildah,projectatomic/buildah,TomSweeneyRedHat/buildah