Run image ENTRYPOINT should be specified in the stack's run image, not by the exporter

[sclevine]

We should remove this: https://github.com/buildpack/spec/blob/master/buildpack.md#phase-4-export
And add a section to the platform spec describing how the stack sets ENTRYPOINT.

(We should change this back if we ever make the export phase responsible for copying the launch-responsible lifecycle component to the final image.)

[nebhale]

Concur. Seems like it's generic enough that baking it into the stack rather than requiring it as part of each and every export is the way to do it. Seems like the right part of the spec as well, not just an implementation detail of how we handle it.

[nebhale]

In the name of consistency between the builder and the launcher in the image that was created, launcher should be contributed by and the ENTRYPOINT set by the builder.

[nebhale]

Requesting @sclevine submit a PR with spec changes.

[sclevine]

This was implemented in the lifecycle and intentionally left unspecified.