Work around a CI bug in GitLab

The latest GitLab release has a bug where i386 docker images can not be downloaded unless the platform is explicitly passed in. See https://gitlab.com/gitlab-org/gitlab/-/issues/504433

This change adds in that explicit platform and fixes the .deb jobs.

There is one CI job that is still broken, but it lives in the signet-client repo, and needs to be fixed up there. After that is updated, a signet-client release will have to be cut, and this repo will need to be updated to build said new release.