This repository periodically (currently once per month) creates a Debian Stretch minimal root filesystem for every architecture that Debian supports (by calling debootstrap). I'm currently using these for multi-arch builds on Azure Pipelines (where the packaging takes place), but they should be useful for most scenarios requiring root filesystems for multiple architectures.
It's relatively easy to find root filesystems for most Linux distributions on the internet, including Ubuntu (who make official releases of root filesystems for datacenter use). I prefer working with Debian as I find there is much better coverage of different architectures, and I've found the package naming to be more consistent. As I was unable to find downloaded root filesystem tarballs on the internet, I decided to build some for my own use (but hopefully they're useful to you too!).
prebootstrap uses QEMU and debootstrap's foreign and second-stage options to fully setup the filesystem, which is then tarballed and tagged to a release for that month on Github (the release tab above). This should make it easy to lock versions to a filesystem for a specific month. To download a filesystem with a more recent set of packages just change the month and year in your scripts.
The releases tab holds the downloads, which are tagged and named in the format prebootstrap_RELEASE_VARIANT_ARCHITECTURE_rootfs.tar.gz. As an example, a Debian Stretch minbase armhf root filesystem would be named prebootstrap_stretch_minbase_armhf_rootfs.tar.gz.