Futureproof Dockerfile for new versions of buildkit

[Subbarker]

Description

Kokoro pins an older version of buildkit, so these changes haven't been necessary.

Experimentation with newer versions of buildkit breaks because ironically, they are trying to support OTEL. We need to unset the env vars that buildkit sets or we get errors like: java.net.MalformedURLException: unknown protocol: unix

Related issue

How has this been tested?

Checklist:

• Unit tests
  • Unit tests do not apply.
  • Unit tests have been added/modified and passed for this PR.
• Integration tests
  • Integration tests do not apply.
  • Integration tests have been added/modified and passed for this PR.
• Documentation
  • This PR introduces no user visible changes.
  • This PR introduces user visible changes and the corresponding documentation change has been made.
• Minor version bump
  • This PR introduces no new features.
  • This PR introduces new features, and there is a separate PR to bump the minor version since the last release already.
  • This PR bumps the version.