This Python example is just a simple Flask web application powered by Gunicorn. We pass down GIT_SHA as a Docker build arg as a environment variable and expose it as X-App-Version header using an after_request hook in Flask.
The Dockerfile first copies just the requirements.txt file and install this via pip, if we skipped this and just did a COPY . . we'd be missing Docker's caching layers and waiting for a pip install everytime we change something inside our directory.