[CI] Fix multiarch builds

[serban-nicusor-toptal]

Submission Checklist

• Run unit tests
• Documentation
  • If a user-facing facing change was made, the documentation PR is here:
  • OR, no user-facing changes were made

Release notes

Add an optional stage to build the multiarch with a custom docker tag.
Add a variable to hold the docker tag used for multiarch builds, so developers can change it when making changes to the docker image in a PR.
Eliminate the usage of --privileged for the multiarch builds themselves.
Lock multiarch build & execution on linux && triqs agent to ensure permissions are correct across runs.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[serban-nicusor-toptal]

All changes are here I think, I'm now moving into testing this to make sure everything still works as expected.

[serban-nicusor-toptal]

Hey @WardBrian this is ready for a review.
I ran 34 and 35 to make sure it build and the second run won't get permission errors.
https://jenkins.flatironinstitute.org/blue/organizations/jenkins/Stan%2FStanc3/detail/PR-1409/34/pipeline
https://jenkins.flatironinstitute.org/blue/organizations/jenkins/Stan%2FStanc3/detail/PR-1409/35/pipeline

Feel free to give it a few test runs. I've updated the description with the changes too.

[WardBrian]

Thanks! Are there links to runs of each of these steps working?

[serban-nicusor-toptal]

@WardBrian this is ready now, see my comment above with the two jenkins urls, those are successful builds, one after another to ensure we do not run into the same error as before.

[WardBrian]

Those links both seem to go to builds where the Build and push multiarch docker… is grayed out

[serban-nicusor-toptal]

Oh my bad, I thought you were looking for the actual multiarch builds based on this image.
See: https://jenkins.flatironinstitute.org/blue/organizations/jenkins/Stan%2FStanc3/detail/PR-1409/29/pipeline