How to create the smallest possible Docker image after installing apt dependencies

Question

I've created a Docker image using debian as the parent image. In my Dockerfile I've installed some dependencies using apt and pip.

Now, I want to get rid off everything that is not completely necessary to run my app, which of course, needs the dependencies installed.

For now I have the following lines in my Dockerfile after installing the dependencies.

RUN rm -rf /var/lib/apt/lists/* \
    && rm -Rf /usr/share/doc && rm -Rf /usr/share/man \
    && apt-get clean

I've also installed the dependencies using the --no-install-recommends option.

Anything else I can do to reduce the footprint of my Docker image?

PS: just in case, this is how I installed the dependencies:

RUN apt-get update \
    && apt-get install -y --no-install-recommends \
       sudo systemd \
       build-essential libffi-dev libssl-dev \
       python-pip python-dev python-setuptools python-wheel

Answer 1

To reduce the size of the image, you need to combine your RUN commands into one. When you create files in one layer and delete them in another, the files still exist on the drive and are shipped over the network. Their existence is just hidden when the layers of the filesystem are assembled for your container.

The Dockerfile best practices explain this in more detail: https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#run

I'd also recommend building with docker build --rm=false --no-cache . (temporarily) and then reviewing the output of docker diff on each of the created images to see what files are created in each layer.