[CI] Add ref checkout for scheduled docker builds (backport #461)

[mergify]

This is an automatic backport of pull request #461 done by Mergify.
Cherry-pick of a6374dd has failed:

On branch mergify/bp/humble/pr-461
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit a6374dd.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   .github/workflows/humble-docker-build.yaml

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   .github/workflows/iron-docker-build.yaml
	deleted by us:   .github/workflows/rolling-docker-build.yaml

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

---

Mergify commands and options

More conditions and actions can be found in the documentation.

You can also trigger Mergify actions by commenting on this pull request:

• @Mergifyio refresh will re-evaluate the rules
• @Mergifyio rebase will rebase this PR on its base branch
• @Mergifyio update will merge the base branch into this PR
• @Mergifyio backport <destination> will backport this PR on <destination> branch

Additionally, on Mergify dashboard you can:

• look at your merge queues
• generate the Mergify configuration with the config editor.

Finally, you can contact us on https://mergify.com

[mergify]

Cherry-pick of a6374dd has failed:

On branch mergify/bp/humble/pr-461
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit a6374dd.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   .github/workflows/humble-docker-build.yaml

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   .github/workflows/iron-docker-build.yaml
	deleted by us:   .github/workflows/rolling-docker-build.yaml

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally