use ubuntu base for buildkit image

[crazy-max]

This sets ubuntu as default base for the buildkit image. This will allow us to setup CDI within the container for NVIDIA devices that require glibc for client librairies.

Last commit publish both alpine and ubuntu variants using the following format:

• moby/buildkit:<tag>: ubuntu root
• moby/buildkit:<tag>-rootless: ubuntu rootless
• moby/buildkit:<tag>-alpine: alpine root
• moby/buildkit:<tag>-rootless-alpine: alpine rootless

Let me know if we should also have an tag alias for debian like moby/buildkit:<tag> = moby/buildkit:<tag>-ubuntu