Minimizing layers in Docker - Commands & Configuration

Introduction

Docker images are made of layers created by each command in a Dockerfile. Minimizing layers helps keep images smaller and faster to download and run.

When you want your Docker images to use less disk space on your computer or server

When you need to speed up the time it takes to build and push images to a registry

When you want to reduce the time it takes to pull images on other machines

When you want to keep your Docker images clean and easier to manage

When you want to avoid unnecessary layers that increase image size without adding value

Config File - Dockerfile

Dockerfile

FROM python:3.11-slim

# Combine update and install in one RUN to minimize layers
RUN apt-get update && apt-get install -y curl && rm -rf /var/lib/apt/lists/*

# Set working directory
WORKDIR /app

# Copy application code
COPY app.py /app/app.py

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

FROM: sets the base image.

RUN: combines update and install commands with cleanup in one layer to reduce image size.

WORKDIR: sets the working directory inside the container.

COPY: adds application code.

CMD: defines the command to run the app.

Commands

Builds the Docker image named 'my-app' with tag '1.0' using the Dockerfile in the current directory. This creates the image with minimized layers as defined.

Terminal

docker build -t my-app:1.0 .

Expected Output

Sending build context to Docker daemon  3.07kB

Step 1/6 : FROM python:3.11-slim

 ---> 3a1b2c3d4e5f

Step 2/6 : RUN apt-get update && apt-get install -y curl && rm -rf /var/lib/apt/lists/*

 ---> Using cache

 ---> 7f8e9d0a1b2c

Step 3/6 : WORKDIR /app

 ---> Using cache

 ---> 1a2b3c4d5e6f

Step 4/6 : COPY app.py /app/app.py

 ---> Using cache

 ---> 9a8b7c6d5e4f

Step 5/6 : CMD ["python", "app.py"]

 ---> Using cache

 ---> 0f1e2d3c4b5a

Successfully built 0f1e2d3c4b5a

Successfully tagged my-app:1.0

Shows the details of the built image to verify its size and tag.

Terminal

docker images my-app:1.0

Expected Output

REPOSITORY   TAG      IMAGE ID       CREATED         SIZE

my-app       1.0      0f1e2d3c4b5a   2 minutes ago   120MB

Displays the layers of the image to confirm that the number of layers is minimized.

Terminal

docker history my-app:1.0

Expected Output

IMAGE          CREATED         CREATED BY                                      SIZE      COMMENT

0f1e2d3c4b5a   2 minutes ago   CMD ["python" "app.py"]                        0B        

1a2b3c4d5e6f   2 minutes ago   WORKDIR /app                                    0B        

9a8b7c6d5e4f   2 minutes ago   COPY app.py /app/app.py                          1.2kB     

7f8e9d0a1b2c   2 minutes ago   RUN apt-get update && apt-get install -y curl…  45MB      

3a1b2c3d4e5f   3 days ago      FROM python:3.11-slim                            75MB      

Key Concept

Each command in a Dockerfile creates a layer, so combining commands reduces the total layers and image size.

Common Mistakes

Using multiple RUN commands for related tasks like update and install separately

Each RUN creates a new layer, increasing image size unnecessarily

Combine related commands with && in a single RUN to create one layer

Not cleaning up temporary files after package installation

Temporary files remain in the layer, making the image larger

Remove cache or temporary files in the same RUN command after installation

Summary

Docker images are built in layers, one per command in the Dockerfile.

Combining commands like update and install in one RUN reduces layers and image size.

Cleaning up temporary files in the same RUN command keeps images smaller and cleaner.