Docker: Layers and Caching

Why does your Docker build take 10 minutes? Because you are ignoring layers.

The Cake Analogy

A Docker image is like a cake with layers. 1. Ubuntu Base 2. Python Install 3. Requirements Install 4. Code Copy

If you change Layer 4 (Code), Docker re-uses Layers 1-3. Instant build! But if you swap Layer 3 and 4...

Bad Dockerfile:

COPY . .
RUN pip install -r requirements.txt

Every time you change a code file (. changes), looking at Layer 1 implies Layer 2 needs to re-run. pip install runs every time. 😭

Good Dockerfile:

COPY requirements.txt .
RUN pip install -r requirements.txt
COPY . .

Now, pip install only runs if requirements.txt changes.

Conclusion

Order matters. Put the things that change least (OS, Dependencies) at the top. Put the things that change most (Code) at the bottom.