fix warnings during docker build

[Iceber]

What type of PR is this?

What this PR does / why we need it:

$ make images
...
 2 warnings found (use docker --debug to expand):
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 2)
 - InvalidDefaultArgInFrom: Default value for ARG ${BUILDER_IMAGE} results in empty or invalid base image name (line 2)
...

https://docs.docker.com/reference/build-checks/invalid-default-arg-in-from/
https://docs.docker.com/reference/build-checks/from-as-casing/
Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[clusterpedia-bot]

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[Copilot]

Pull Request Overview

This PR updates the Dockerfile to suppress build-time warnings by providing a default builder image and correcting keyword casing.

• Adds a non-existent default for BUILDER_IMAGE to avoid the InvalidDefaultArgInFrom warning
• Updates the FROM ... as builder line to use uppercase AS to satisfy the FromAsCasing checker
• Includes comments explaining the chosen defaults and warning references

Comments suppressed due to low confidence (2)

Dockerfile:5

• [nitpick] It may not be immediately clear that changing as to uppercase AS fixes the FromAsCasing warning. Consider adding a brief comment explaining that requirement or merging it with the existing comment block.

FROM --platform=$BUILDPLATFORM ${BUILDER_IMAGE} AS builder

Dockerfile:4

• The quotes around clusterpedia.io/builder:not-build become part of the default value, leading to an invalid image name. Remove the quotes so the ARG reads ARG BUILDER_IMAGE=clusterpedia.io/builder:not-build.

ARG BUILDER_IMAGE="clusterpedia.io/builder:not-build"