Tips to reduce docker size

A Docker layer represents a set of files or changes that are stacked on top of each other to form a complete Docker image. Each layer is built from instructions in the Dockerfile, such as RUN, COPY, and ADD. Each of these instructions creates a new layer in the Docker image.

1) Reduce the number of layers

In a Dockerfile, each FROM, RUN, and COPY command creates a separate layer, increasing the overall size and build time of the image.

To reduce the size of a Docker image, execute multiple commands in a single RUN or COPY instruction to minimize the number of layers in the Dockerfile.

FROM ubuntu:latest
RUN apt update -y
RUN apt install unzip -y
RUN apt install curl -y
RUN apt install python3 -y

Instead of using separate instructions for each command, combine them together:

FROM ubuntu:latest
RUN apt update -y && \
apt install unzip -y && \
apt install curl -y && \
apt install python3 -y

It manage to reduce by 2MB

2) Use smaller base image

The most obvious way to reduce the size of a Docker image is by using a smaller base image.

If you want to create an image for a Python application, consider using the python:3.9-slim image instead of python:3.9.

The size of python:3.9 is about 1.3 GB, while python:3.9-slim is only about 1 GB.

You can further reduce the image size by using the Alpine version. Alpine images are specifically designed to run as containers and are very small. The python:3.9-alpine image is only 49 MB.

3) use no install recommends flag

When we run the apt install command to install some packages, it installs some unnecessary recommended packages. Using the --no-install-recommends flag can significantly reduce the image size.

FROM ubuntu:latest
RUN apt update -y && \
apt install unzip -y --no-install-recommends && \
apt install curl --no-install-recommends -y && \
apt install python3 -y --no-install-recommends

Reduce by 3MB

4) Remove the package after installed

After apt install, we can use this commands (rm -rf /var/lib/apt/lists/*)to remove the package left in storage to reduce the image size

FROM ubuntu:latest
RUN apt update -y && \
apt install unzip -y --no-install-recommends && \
apt install curl --no-install-recommends -y && \
apt install python3 -y --no-install-recommends && \
rm -rf /var/lib/apt/lists/*

It reduce alot more this time.

5) Use .dockerignore

If you do not want to copy certain files into the Docker image, then using a .dockerignore file can save you some space.

There are some hidden files/folders in the build context that you can transfer to the image using the ADD or COPY commands, such as .git, etc. Including a .dockerignore file to reduce the size of the Docker image is a good practice.

Example of a .dockerignore file.

ignorethisfile.txt
logs/
ignorethisfolder/
.git
.cache
*.md

6) Copy after RUN

In some cases, you make minor changes to your code and need to repeatedly build the image from the Dockerfile. In such cases, placing the COPY command after the RUN command will help reduce the image size, as Docker will be able to make better use of its caching capabilities in this scenario.

It will create a cache for the image with installed dependencies, and each time the code changes, Docker will use this cache to create the image. This will also reduce the Docker build time.

FROM ubuntu:latest
RUN apt update -y && \
apt install unzip -y --no-install-recommends && \
apt install curl --no-install-recommends -y && \
apt install python3 -y --no-install-recommends && \
rm -rf /var/lib/apt/lists/*
COPY file /home/ubuntu

7) Delete the software package

If you need to install some packages in a Docker image and you are downloading them from external sources, it is best to delete those packages after installation.

For example, if you want to install AWS CLI V2 from a zip file, remember to also delete the zip file after successful installation.

FROM ubuntu:latest
RUN curl "https://awscli.amazonaws.com/awscli-exe-linux-x86_64.zip" -o "awscliv2.zip" && \
unzip awscliv2.zip && \
sudo ./aws/install && \
rm awscliv2.zip

Author

Ik0nw

Interview question

Can you compress this dockerfiles to make it smaller?

For python project what might be the files that might add it into .dockerignore ?

craft a sample .dockerignore file.

# Python images
FROM python:3.9

# Copying all current files to /app
COPY . /app

# update and install package
RUN apt-get update
RUN apt-get install -y curl
RUN apt-get install -y git

# Install python flask
RUN pip install flask
RUN pip install requests

# Install the aws cli
RUN curl "https://awscli.amazonaws.com/awscli-exe-linux-x86_64.zip" -o "awscliv2.zip"
RUN unzip awscliv2.zip
RUN sudo ./aws/install

# Setting the working directory
WORKDIR /app

# Running the application
CMD ["python", "app.py"]