Reducing the size of containers

A common concern of new Docker users is the size of their container images. It's true that containers provide a lot of space overhead compared to plain Python packages, but it is usually nothing if we compare the size of images for virtual machines. However, it is still very common to host many services on a single virtual machine, but with a container-based approach, you should definitely have a separate image for every service. This means that with a lot of services, the overhead may become noticeable.

If you want to limit the size of your images, you can use two complementary techniques:

1. Use a base image that is designed specifically for that purpose: Alpine Linux is an example of a compact Linux distribution that is specifically tailored to provide very small and lightweight Docker images. The base image is only 5 MB in size, and provides an elegant package manager that allows you to keep your images compact, too.
2. Take into consideration the characteristics of the Docker overlay filesystem: Docker images consist of layers where each layer encapsulates the difference in the root filesystem between itself and the previous layer. Once the layer is committed the size of the image cannot be reduced. This means that if you need a system package as a build dependency, and it may be later discarded from the image, then instead of using multiple RUN instructions, it may be better to do everything in a single RUN instruction with chained shell commands to avoid excessive layer commits.

These two techniques can be illustrated by the following Dockerfile:

# Here we use bare alpine to illustrate
# package management as it lacks Python
# by default. For Python projects in general
# the 'python:3.7-alpine' is probably better
# choice.
FROM alpine:3.7

# Add python3 package as alpine image lacks it by default
RUN apk add python3

# Run multiple commands in single RUN instruction
# so space can be reclaimed after the 'apk del py3-pip'
# command because image layer is committed only after
# whole whole instruction. 
RUN apk add py3-pip && \
    pip3 install django && \
    apk del py3-pip

# (...)