Add observer Docker build validation to CI #687

CI runs observer tests against the full repository checkout, but does not build tools/observer/Dockerfile. This leaves missing Docker build inputs undetected.

For example, the observer imports the operator's pkg/topology package, but its Dockerfile omitted pkg/. Tests passed while make observer-build failed, also breaking make kind-deploy. PR #682 fixes that omission.

Add a single-architecture CI check that builds the observer image so similar regressions are caught before merging.

Acceptance criteria

• Build tools/observer/Dockerfile using the repository root as the build context.
• Run the check on pull requests and pushes to main.
• Fail the check if the observer image cannot be built.
• Build for one architecture, without pushing an image or requiring registry credentials.
• Confirm the check catches the missing pkg/ failure fixed by fix(observer): copy root packages into image builder #682.

Priority

Low-priority maintenance follow-up. This adds regression coverage and is not a prerequisite for merging #682. Keep CI cost modest; a cluster deployment or multi-architecture build is not needed for this check.