Fix Dockerfile ARG continuation parsing

[SimonYang (simonyang08)]

Fixes #1286

Summary

• join Dockerfile lines that use the default backslash continuation before scanning instructions
• collect every declaration from a continued ARG instruction
• preserve the existing single-declaration behavior for ENV and USER
• add regression coverage for default and buildArgs-overridden values

Root cause

The lightweight Dockerfile scanner matched physical lines and recorded only one name/value pair for each ARG instruction. Docker joins continuation lines before parsing, so the second declaration was invisible to the CLI even though Docker/BuildKit accepted it. A later FROM reference therefore resolved to an empty value and produced an invalid image reference.

Validation

• focused dockerfileUtils suite: 70 passing
• npm run type-check: passed
• npm run lint: passed
• git diff --check: passed

The repository-wide npm test script uses Unix env/rm commands and is not directly runnable from the current Windows shell. The focused parser suite covers the changed module; upstream CI should provide the remaining platform and integration validation.

AI disclosure

This change was prepared with OpenAI Codex assistance and reviewed and validated locally by the contributor.

[SimonYang (simonyang08)]

@microsoft-github-policy-service agree