Reduce the size of your builder images

Docker images should have a small size. For example, Alpine Linux images have a mere 50MB of size, debian-slim is 50-60MB of size. Why do we seek a small size footprint? One reason is simply because the docker images are usually pushed to, and download from, docker repositories like dockerhub or other private docker repositories.

Use a small base image

Alpine and debian-slim-stable are like 50MB. Use these kind of images instead of bigger images.

Copy only the required files

Use COPY with care, and always make use of a .dockerignore to avoid copying folders and files that the image does not need but that are heavy in regards of space.

Instead of doing

COPY . .

Copy individual files and folders

COPY src/ .

A common dockerignore for a python project could be the following:

.git
build
coverage
tmp
venv
.idea
.vscode

Reduce the number of docker layers

Each docker layer occupies a certain amount of space, by reducing them you can achieve some reduction of the size of the docker image.

Instead of:

RUN apt update
RUN apt upgrade
RUN apt install -y curl

Do:

RUN apt update && \
    apt upgrade && \
    apt install -y curl

Remove caches, logs and temporal information

Try to remove all information that is not

# Install a package, don't install more packages than the required ones
# and clean the apt cache and remove orphaned packages.
RUN apt update && \
    apt upgrade && \
    apt install -y --no-install-recomends curl && \
    apt clean && \
    apt autoremove

Conclusion

There are other ways to reduce the size of docker images, like using dive to check the size of each docker layer, and of course the use of the builder pattern, that we will shown in the future.

However, this post has shown some basic advice to reduce the size of docker images, and that was my intent here.