include on-demand post install-script

[mallej]

Problem: At the moment an On-Demand package that is created with outset-pkg has no on-demand trigger included.

Describe the solution you'd like
If the build process of outset-pkg --target on-demand would include a post install-script that triggers the on-demand process the packe would be more self-contained.

Describe alternatives you've considered
an option to outset-pkg to include a individual script would also help.

Additional context
I think the original Outset included this feature.
https://github.com/chilcote/outset/blob/master/custom-ondemand/scripts/postinstall