ci: add GitHub Actions workflow and GHCR Docker image build

[mrmeaow]

Motivation

• Add a reproducible CI pipeline to validate workspace builds and produce a Docker image for publishing to GitHub Container Registry.
• Provide a production-oriented multi-stage Dockerfile and a lightweight startup script so the monorepo can be run inside a container for local testing and CI.
• Reduce Docker build context noise and document Docker/CI usage in the repository README.

Description

• Added a GitHub Actions workflow at .github/workflows/ci-image.yml that runs install and pnpm build on PRs and builds/pushes a Docker image to GHCR on pushes/tags.
• Added a multi-stage Dockerfile that installs dependencies, builds the web app, and assembles a runtime image that runs the API and SSR web together.
• Added .dockerignore to exclude node_modules, build artifacts, and other unnecessary files from the Docker context.
• Added scripts/start.sh to launch the server and SSR web inside the container, and updated README.md with Docker and CI usage instructions.

Testing

• Ran pnpm build in the workspace and it completed successfully.
• Attempted to run docker build -t devsms:local . locally, but the Docker CLI was not available in this environment so the image build could not be executed here.
• The new GitHub Actions workflow has been added to the repo and will run in CI on PRs and pushes; it was not executed in this local environment as part of these changes.

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Codex Task