No feedback on Containerfile directives that do not work with bootc

[stefwalter]

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.