Build an Optimized Python Image

Learning objectives

• Build the app image
• Convert to a multi-stage Dockerfile
• Reduce the final image size
• Run as a non-root user
• Add a working HEALTHCHECK

Before you start

Required software: Docker Engine 24+

Prerequisites: Docker Fundamentals path

1. Download and unzip the lab.
2. Build the starter image and note its size: docker build -t pyapp:before starter/ && docker images pyapp:before.

Instructions

Hints

💡 Where does the size come from?

Build tooling (compilers, caches, dev headers) shouldn’t reach the runtime image. A builder stage that produces a wheel/venv, copied into a slim final stage, removes most of it.

💡 Health check keeps failing

HEALTHCHECK runs inside the container — the tool it calls (e.g. curl) must exist in the final image, or use Python itself to probe.

Validate your work

From the lab folder, run:

bash validation/validate.sh

The script reads only your local Docker state, never transmits data off your machine, and returns a status code:

• 0 — Lab successfully completed
• 1 — Validation failed
• 2 — Required dependency missing
• 3 — Lab environment not running
• 4 — Configuration error
• 5 — Validation timed out

Note: validation targets Linux first. On Docker Desktop (macOS/Windows) some host-level checks (e.g. disk usage) may report differently; each script documents its limitations.

Solution

Reset the lab

• docker compose down -v && docker compose up -d from the original lab files restores the starting (broken) state.

Interview questions

• What is the difference in a `RUN` layer between `pip install` in a builder vs the final stage?
• Why run containers as non-root?
• What makes a good container HEALTHCHECK?