ci: add release branch docker tags (backport #8635)

[mergify]

Description

closes: #XXXX

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Linked to GitHub issue with discussion and accepted design, OR link to spec that describes this work.
• Include changelog entry when appropriate (e.g. chores should be omitted from changelog).
• Wrote unit and integration tests if relevant.
• Updated documentation (docs/) if anything is changed.
• Added godoc comments if relevant.
• Self-reviewed Files changed in the GitHub PR explorer.
• Provide a conventional commit message to follow the repository standards.

---

This is an automatic backport of pull request #8635 done by [Mergify](https://mergify.com).

[graphite-app]

The empty steps.reltag.outputs.full_tag value when not on a release branch could cause build errors. Consider using a conditional expression to safely handle this case:

tags: |
  ${{ steps.meta.outputs.tags }}
  ${{ steps.reltag.outputs.full_tag != '' && steps.reltag.outputs.full_tag || '' }}

This ensures the tag is only included when it's defined, preventing potential errors on non-release branches.

Suggested change

	tags: |
	${{ steps.meta.outputs.tags }}
	${{ steps.reltag.outputs.full_tag }}
	tags: |
	${{ steps.meta.outputs.tags }}
	${{ steps.reltag.outputs.full_tag != '' && steps.reltag.outputs.full_tag || '' }}

Spotted by Diamond



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