With the multi-stage Dockerfile complete, building specific stages is done using the --target flag.
Building the Dev Image
docker build -f Dockerfile --target dev -t lean-dev .This builds all stages up to and including dev. The resulting image is approximately 385 MB—significantly smaller than the 2 GB naive approach, despite including debug tools and dev dependencies.
Building the Production Image
docker build -f Dockerfile --target prod -t lean-prod .If the dev image was built first, Docker reuses cached layers from python_base and builder. The production build completes almost instantly, only needing to:
- Create the non-root user
- Copy the virtual environment from the builder
- Copy the source code
The production image weighs in at approximately 198 MB.
Stage Hierarchy
| Stage | Purpose | Included In Final Image |
|---|---|---|
python_base | Alpine base, Python optimizations | Configuration only |
builder | Create virtual environment | .venv copied to dev/prod |
dev | Local development with debug tools | ~385 MB |
prod | Minimal production image | ~198 MB |
Build Caching
Docker's layer caching makes subsequent builds fast. As long as pyproject.toml and uv.lock haven't changed, the builder stage is cached. Only changes to source code trigger a rebuild of the final COPY layer—typically completing in seconds.