[CI:DOCS] Document how entrypoint is configured in buildah config

[rhatdan]

Fixes: #5598

What type of PR is this?

/kind api-change
/kind bug
/kind cleanup
/kind deprecation
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake
/kind other

What this PR does / why we need it:

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Which issue(s) this PR fixes:

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Does this PR introduce a user-facing change?

None

[packit-as-a-service]

We were not able to find or create Copr project packit/containers-buildah-5734 specified in the config with the following error:

Packit received HTTP 500 Internal Server Error from Copr Service. Check the Copr status page: https://copr.fedorainfracloud.org/status/stats/, or ask for help in Fedora Build System matrix channel: https://matrix.to/#/#buildsys:fedoraproject.org.

---

Unless the HTTP status code above is >= 500, please check your configuration for:

1. typos in owner and project name (groups need to be prefixed with @)
2. whether the project name doesn't contain not allowed characters (only letters, digits, underscores, dashes and dots must be used)
3. whether the project itself exists (Packit creates projects only in its own namespace)
4. whether Packit is allowed to build in your Copr project
5. whether your Copr project/group is not private

[nalind]

added executed is missing a word in the between them? containers should be container's

[rhatdan]

@ makhomed @gcb PTAL

[rhatdan]

@nalind @flouthoc @giuseppe @TomSweeneyRedHat @lsm5 @mheon PTAL

[flouthoc]

LGTM, needs a rebase though

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