fix(ci): validate Docker image build on PR Docker changes

[SFangYy]

Description

Run a Docker build validation job on pull requests when Dockerfile or .build-config.yml changes, without pushing the image.

Also keep /tmp/build-config.yml until the Node installation step finishes so the Docker build can still resolve the configured Node version after the Verilator step.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the style guidelines of this project
• I have added the appropriate labels
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[Copilot]

Pull request overview

Adds pull-request-time Docker image build validation when Docker-related files change, and adjusts the Dockerfile so .build-config.yml remains available long enough for Node version resolution during the image build.

Changes:

• Add CI jobs to detect PR changes to Dockerfile / .build-config.yml and run a non-pushing Docker build as validation.
• Update Dockerfile cleanup timing so /tmp/build-config.yml is removed after Node installation instead of after the Verilator step.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
Dockerfile	Delays removal of /tmp/build-config.yml until after Node install so later build steps can still read configured versions.
.github/workflows/ci.yml	Adds PR-only change detection and a Docker build-only validation job gated on Docker-related diffs.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.