fix: release build scripts (backport #1286)

[mergify]

This is an automatic backport of pull request #1286 done by Mergify.
Cherry-pick of 2029415 has failed:

On branch mergify/bp/v7.x/pr-1286
Your branch is up to date with 'origin/v7.x'.

You are currently cherry-picking commit 2029415.
  (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:   .build.sh
	modified:   Makefile
	modified:   contrib/images/Makefile
	modified:   contrib/images/rbuilder/Dockerfile
	new file:   contrib/images/rbuilder/README.md
	modified:   contrib/images/rbuilder/buildlib.sh

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.md

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

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