Export path variables

Any variable macros set from within a Makefile is available within the Makefile. Environment variables are also read in as variables and will be re-exported to subshells. New variables defined within the Makefile, such as PKG_PATH, are however not exported per default and so will not be visible to a subshell. The expansion of the include manifests into the Dockerfile uses a subshell and requires this be exported.

We could also explicitly set it just for the Dockerfile target but I figure there might be other targets in the future that might want to access these values.

Closes #125 (closed)