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 commits publish both alpine and ubuntu variants using the following format:

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

[Copilot]

Pull Request Overview

This PR switches the default base image for Buildkit to Debian and differentiates between Debian and Alpine variants.

• Updated the workflow matrix to include a new "base" key with values for Debian (empty string) and Alpine.
• Adjusted environment variable configurations accordingly to use the new matrix keys.

Files not reviewed (3)

• Dockerfile: Language not supported
• docker-bake.hcl: Language not supported
• hack/images: Language not supported

Comments suppressed due to low confidence (2)

.github/workflows/buildkit.yml:191

• [nitpick] The matrix key 'base' may not clearly indicate that an empty string represents Debian. Consider using a more descriptive name such as 'imageBase' to clarify intent.

+        base:

.github/workflows/buildkit.yml:192

• [nitpick] It might be helpful to add a comment explaining that an empty string for 'base' indicates the default Debian image, improving maintainability.

+          - ''

[crazy-max]

Ubuntu rootless doesn't build with uid 1000 as it already exists. Wonder if we should change it for both Alpine and Ubuntu to be consistent 🤔

https://github.com/moby/buildkit/actions/runs/16194070978/job/45717191384?pr=5944#step:7:1930

 > [linux/amd64 rootless-ubuntu 3/3] RUN adduser --disabled-password --gecos "" --uid 1000 user   && mkdir -p /run/user/1000 /home/user/.local/tmp /home/user/.local/share/buildkit   && chown -R user /run/user/1000 /home/user   && echo user:100000:65536 | tee /etc/subuid | tee /etc/subgid:
1.126 fatal: The UID 1000 is already in use.

Edit: Skipped user creation and use one already created from base image.