Add CI job to test Docker builds and verify image size

[adnaan]

Context

This follow-up task was identified during the review of PR #132.

Source PR: #132
PR Title: feat: add Docker support with multi-stage build
Suggested by: @claude[bot]

Task Description

Add automated Docker build testing to the CI pipeline to:

1. Verify the Dockerfile builds successfully
2. Check that the image size stays under 50MB target
3. Optionally run basic smoke tests on the container

Acceptance Criteria

• CI workflow includes Docker build step
• Image size is validated (< 50MB)
• Build failures are reported clearly

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This issue was automatically created from PR review comments.