fix: partial support multi-platform build with automatic platform args

[tky2240]

Summary

This PR fix missing platform arguments (e.g., $TARGETARCH) for multi-platform builds.

Key Changes

• Fixed Argument Substitution: Platform arguments are now resolved within the findBaseImage function based on the CLI execution environment or --platform args.
• Added Base Image Validation: The build will now fail with a error if the base image resolves to a different across the platforms.

How to Test

• ✅ Confirm a multi-platform build succeeds when using a base image with a platform variable (e.g., FROM ${TARGETARCH}-base).
  • src/test/configs/dockerfile-with-automatic-platform-args/
• ❌ Verify that a build fails as expected when using base images that are inconsistent across platforms.
  • src/test/configs/dockerfile-with-inconsistent-base-image/

Related Issus

• Automatic platform ARGs are not recognized in a FROM stage for extension version >= 0.255.2 (exclude pre-release version) microsoft/vscode-remote-release#7582
• Remote-containers fails to build the container when using TARGETARCH substitution for stages microsoft/vscode-remote-release#9605
• findBaseImage in dockerfileUtils.ts does not handle buildkit automatic variables in layers name #275

Note

• This PR is out of scope for Docker Compose.

[tky2240]

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