Improve image reproducibility

Relates to #371 (closed)

Identify sources of non-reproducibility. Make images (especially base images) reproducible if possible.

Investigate:

• buildah bud --squash
• buildah bud --timestamp=0
• Identify non-reproducible labels (per-build timestamps or ids)
• Clear dnf/yum cache dnf clean all && rm -rf /var/cache/yum (this should already be done but verify on base images)
• Anything else? do a rebuild and diff the images? Use a container diff tool.
• Ensure we delete added installers from images

https://github.com/containers/buildah/blob/main/docs/buildah-build.md