Publicly support COMPOSE_UNPACKER_IMAGE

[jaap3]

Is your feature request related to a problem? Please describe

The discussion about pre/post script hooks has multiple people (myself included) pointing out that the COMPOSE_UNPACKER_IMAGE environment variable can be used to replace the default compose-unpacker image. This allows users to provide a custom image to support their specific use cases.

Describe the solution you'd like

I'd like to see COMPOSE_UNPACKER_IMAGE officially documented and supported. That way, I can trust that my workaround will continue to work with future versions of Portainer.

Describe alternatives you've considered

I did consider submitting a patch to add my needed feature directly to the unpacker image. However, since there's no guarantee that such a feature would be accepted, having a supported "escape hatch" via COMPOSE_UNPACKER_IMAGE is important for the foreseeable future.

Additional context

For context, I currently maintain a patch set that adds a webhook feature to Portainer's unpacker, which lets me process the checkout before the stack is started:
WRIJ/patch-portainer-unpacker