Fix Multi-stage Build Interdependencies

[ricejasonf]

When there are multiple dependencies specified for a build stage, each dependency should install to both /opt/install and /opt/sysroot so that is usable by subsequent dependencies (which is the entire point).

It's not clear to me if we need to install these to /opt/install at all since they exist only for the final dependency. Does the final program need access to the artifacts or can we default to making them private?

Currently deps that are docker images just copy to both.