How to optimize Dockerfile instructions for faster build times

You can optimize your Dockerfiles for faster build times by leveraging build cache, reducing build context, and more. This tutorial goes over best practices to follow when creating Dockerfiles.

Prerequisites

You should have Docker installed. Get Docker for your operating system if you haven't already.

1. Use a smaller base image

First, you can start with a smaller base image to create minimal images. This reduces the overall size of the Docker image and speeds up the build process.

For example, when containerizing Python applications, you can start with a python:3.x-slim image, a smaller version of python:3.xwhich contains only the essential components needed to run Python instead of the default python:3.x.

2. Take advantage of Docker's build cache

The order of instructions in a Dockerfile influences build times because of how Docker leverages its build cache.

Docker builds images by sequentially executing instructions in the Dockerfile, creating a new image layer for each instruction. If a layer has not changed since it was last created, Docker can reuse the cached layer, speeding up the build process.

That's why it's important optimize instruction order to maximize cache accesses:

• Place frequently changing instructions at the end: Place frequently changing instructions, such as copying application code, towards the end of the Dockerfile. This reduces the chances of invalidating the cache throughout the build.
• Place instructions that change less frequently at the beginning:Instructions such as installing operating system packages, setting environment variables, and installing dependencies (if the dependencies do not change frequently) should be placed early to maximize cache accesses.

Let's take an example Dockerfile:

# Suboptimal ordering
FROM python:3.11-slim

# Set the working directory
WORKDIR /app

# Copy the entire application code
COPY . .

# Install the required Python packages
RUN pip install --no-cache-dir -r requirements.txt

# Expose the port on which the app runs
EXPOSE 5000

# Run the application
CMD ("python3", "app.py")

In this initial Dockerfile, any changes to the application code will invalidate the cache throughout the build process, including installing dependencies.

Here is the optimized version:

# Better ordering of instructions
FROM python:3.11-slim

# Set the working directory
WORKDIR /app

# Install dependencies
COPY requirements.txt requirements.txt
RUN pip install --no-cache-dir -r requirements.txt

# Copy the current directory contents into the container at /app
COPY . .

# Expose the port on which the app runs
EXPOSE 5000

# Run the application
CMD ("python3", "app.py")

In this optimized Dockerfile, if a change occurs in the application code, Docker can still use the cached layers to install dependencies. This way, changes in the application code do not unnecessarily trigger reinstallation of dependencies.

3. Use multi-stage builds

Multi-stage builds allow you to separate the build environment from the final execution environment, which can reduce the size of the final image by including only the necessary runtime dependencies.

Consider the following Dockerfile with multi-stage compilation:

# Build stage
FROM python:3.11 AS builder
RUN apt-get update && apt-get install -y build-essential
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt

# Final stage
FROM python:3.11-slim
COPY --from=builder /usr/local/lib/python3.11/site-packages /usr/local/lib/python3.11/site-packages
COPY --from=builder /usr/local/bin /usr/local/bin
COPY . /app
WORKDIR /app
CMD ("python3", "app.py")

In this example, the build dependencies are installed on the builder stage, and only the necessary runtime dependencies are copied to the final image.

4. Minimize the build context with .dockerignore files

Make sure you have a .dockerignore file to exclude unnecessary files from being copied into the Docker context, reducing build time. Similar to .gitignoreThis file tells Docker which files to ignore during the build process, which reduces the size of the context.

In it .dockerignore file, may include temporary files, virtual environments, IDE settings, and other unnecessary files that you do not want included in the build context.

From reducing the base image size to optimizing the build context, these optimizations should help make your Docker builds more efficient.

Additional Resources

The following resources should provide you with more information:

twitter.com/balawc27″ rel=”noopener”>girl priya c Bala is a technical developer and writer from India. She enjoys working at the intersection of mathematics, programming, data science, and content creation. Her areas of interest and expertise include DevOps, data science, and natural language processing. She enjoys reading, writing, programming, and drinking coffee! Currently, she is working on learning and sharing her knowledge with the developer community by creating tutorials, how-to guides, opinion pieces, and more. Bala also creates interesting resource overviews and programming tutorials.