Do not use image CMD if user gave ENTRYPOINT

[mheon]

This matches Docker behavior, and seems to make sense - the CMD may have been specific to the original entrypoint and probably does not make sense if it was changed.

While we're in here, greatly simplify the logic for populating the SpecGen's Command. We create the full command when making the OCI spec, so the client should not be doing any more than setting it to the Command the user passed in, and completely ignoring ENTRYPOINT.

Fixes #7115

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[zhangguanzhang]

https://github.com/containers/podman/pull/7256/checks?check_run_id=959383018

[+0215s] # $ /var/tmp/go/src/github.com/containers/podman/bin/podman-remote --url unix:/tmp/podman.nUsXlU exec f99281f82ea12b5d50e5374a5925dd48f96f09e4f42c4bb50df939c3860f6b13 /no/such/command
[+0215s] # executable file not found in $PATH: No such file or directory: OCI runtime command not found error
[+0215s] # [ rc=127 (expected) ]
[+0215s] # $ /var/tmp/go/src/github.com/containers/podman/bin/podman-remote --url unix:/tmp/podman.nUsXlU exec f99281f82ea12b5d50e5374a5925dd48f96f09e4f42c4bb50df939c3860f6b13 rm -f /IsumOdgf06ffBkW1SwMJ
[+0215s] # $ /var/tmp/go/src/github.com/containers/podman/bin/podman-remote --url unix:/tmp/podman.nUsXlU wait f99281f82ea12b5d50e5374a5925dd48f96f09e4f42c4bb50df939c3860f6b13
[+0215s] # timeout: sending signal TERM to command ‘/var/tmp/go/src/github.com/containers/podman/bin/podman-remote’
[+0215s] # [ rc=124 (** EXPECTED 0 **) ]

These lines did not work as expected for some reason https://github.com/containers/podman/blob/master/test/system/075-exec.bats#L31-L33
can come here to discuss this issue #7241

[mheon]

Rebased, should go green now

[mheon]

@baude @vrothberg @rhatdan @TomSweeneyRedHat PTAL

[edsantiago]

LGTM. Thank you for adding a test - the BATS test that caught this is actually blocked by #7116, which may not actually be a bug, so we currently have no system-test coverage for this. If #7116 never gets fixed I'll find a way to re-add that coverage.

[TomSweeneyRedHat]

LGTM
I thought this wasn't going to work right for some Dockerfiles, but I tested a number that I have and they all worked fine.

[rhatdan]

What happens if image has a entrypoint and the command is not nil? Isn't it supposed to still be
entrypoint + command?

[mheon]

In that case, we will append command to entrypoint as normal. This block only handles getting CMD from the image if the user didn't give us a command.

[rhatdan]

/lgtm