No feedback on Containerfile directives that do not work with bootc

I don't know how to solve this, but there's a whole bunch of Containerfile commands that do not work or work differently with bootc containers. As a user, I don't see a warning at build or runtime about these:

* EXPOSE: Has no effect.
* ENTRYPOINT: ```/sbin/init``` should be the ```CMD``` ... and the usual ```CMD``` related to the workload needs to become a systemd service or quadlet?
* STOPSIGNAL: Seems this always needs to be SIGRTMIN+3?
* WORKDIR: When used to place executables or files accessed by a systemd service, fails due to SELinux behaving differently with a container than with a booted host. See #439

Others?

* ENV: Has an effect at buildtime, does it work at runtime?
* HEALTHCHECK
* SHELL
* VOLUME

Again, Not sure how these would work. but the user is seems to be expected to discover these differences.

Start by inventorying the listed Containerfile directives and the differences described for bootc at build and runtime. Read the related discussion in #439, then determine whether each directive needs a warning, documentation, or both; done should clearly identify the supported behavior and user-facing feedback for each case.

Vom Indexierungsmodell aus dem Issue-Text verfasst.