Add GitHub Actions workflow for linting, tests, and Docker build

[dwetmore]

Motivation

• Ensure pull requests and pushes to main run linting, tests, and a Docker build to catch issues early in this FastAPI project.
• Target the application code in app/ and validate the app/Dockerfile can build a container image.

Description

• Add .github/workflows/ci.yml which triggers on pull_request and push to main and defines a build job running on ubuntu-latest.
• The job checks out the repo, sets up Python 3.11 with actions/setup-python@v5, and installs dependencies from app/requirements.txt plus pytest and ruff.
• The workflow runs ruff check app for linting, runs pytest -q (with the committed exit-code handling), and runs docker build -t chatbot-app -f app/Dockerfile app to verify the image builds.

Testing

• No automated tests were executed as part of this change; the workflow file was added and will run on GitHub for future pull requests and pushes to main.

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Codex Task