Chapter 1: Introduction to Efficient Docker Images

In this article, we will explore strategies to enhance your Docker build processes and create more compact images. To illustrate our point, we’ll stick with a healthy theme—think salads over pizza, donuts, or bagels when it comes to slimming down our Docker images.

In Part 3 of this series, we discussed essential Dockerfile instructions. If you haven't read it yet, you can check it out [here](#).

Dockerfile Instructions Cheat Sheet

• FROM: Defines the base image.
• LABEL: Adds metadata, ideal for maintainer details.
• ENV: Establishes a persistent environment variable.
• RUN: Executes a command, creating an image layer; commonly used for installing packages.
• COPY: Transfers files and directories to the container.
• ADD: Similar to COPY but can unpack local .tar files.
• CMD: Specifies a command and its arguments for the running container; only one CMD is allowed.
• WORKDIR: Sets the working directory for subsequent instructions.
• ARG: Defines build-time variables.
• ENTRYPOINT: Similar to CMD but retains arguments for the running container.
• EXPOSE: Indicates a port to be exposed.
• VOLUME: Creates a mount point for persistent data storage.

Now, let’s delve into how to craft our Dockerfiles to maximize efficiency during image development and container deployment.

Leveraging Caching for Efficient Builds

One of Docker's key advantages is its caching mechanism, which allows you to iterate your image builds more swiftly. When constructing an image, Docker processes the instructions in your Dockerfile sequentially, checking for existing intermediate images in its cache that can be reused rather than generating duplicates.

If an instruction invalidates the cache, the subsequent instructions will also trigger new intermediate images. Essentially, if the base image is cached, it will be reused; otherwise, it results in a cache miss.

For instance, if a RUN pip install -r requirements.txt command is present in your Dockerfile, Docker will search for that specific command in its cached intermediate images without comparing the contents of the old and new requirements.txt files. This can create issues if new packages are added to the file and you need to reinstall with the updated names, which I will address shortly.

Unlike other Docker instructions, the ADD and COPY commands require Docker to assess the contents of the referenced files for cache hits. It compares checksums, and any alterations will invalidate the cache.

Here are some effective caching tips:

• Disable caching with --no-cache=True during the build if necessary.
• Place frequently changing instructions toward the bottom of your Dockerfile.
• Chain RUN apt-get update with apt-get install to minimize cache misses.
• Follow this structure when using a requirements.txt file to avoid stale images:

COPY requirements.txt /tmp/

RUN pip install -r /tmp/requirements.txt

COPY . /tmp/

Feel free to share your caching strategies in the comments or on Twitter @discdiver.

Reducing Image Size

Docker images can become quite large, which can slow down deployment and resource usage. Opt for a lighter approach—think salads instead of bagels!

Using an Alpine base image can significantly cut down your image size. Alpine is a minimal Linux distribution, usually under 5 MB, but requires more effort to configure dependencies for your application.

If you need Python, the Python Alpine variant strikes a good balance. For example, an image built with Python Alpine containing a simple print("hello world") script weighs around 78.5 MB. Here’s a sample Dockerfile:

FROM python:3.7.2-alpine3.8

COPY . /app

ENTRYPOINT ["python", "./app/my_script.py", "my_var"]

While the base image appears as 29 MB on Docker Hub, the child image increases in size once Python is installed.

Utilizing Multistage Builds

Another effective way to reduce image size is through multistage builds. This approach allows you to use multiple FROM statements, enabling selective copying of build artifacts and leaving behind unnecessary files.

Each FROM starts a new stage and can utilize a different base image. Here’s a modified example of a multistage build:

FROM golang:1.7.3 AS build

WORKDIR /go/src/github.com/alexellis/href-counter/

RUN go get -d -v golang.org/x/net/html

COPY app.go .

RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o app .

FROM alpine:latest

RUN apk --no-cache add ca-certificates

WORKDIR /root/

COPY --from=build /go/src/github.com/alexellis/href-counter/app .

CMD ["./app"]

Notice how we name the first stage with AS build, which can later be referenced during the COPY step.

While multistage builds are beneficial, they can complicate your Dockerfile, so consider using them primarily for production scenarios.

The Role of .dockerignore

Understanding .dockerignore files is crucial for anyone looking to optimize their Docker images. Similar to .gitignore, this file lists patterns for Docker to exclude when building an image.

Place the .dockerignore file in the same directory as your Dockerfile. When you execute docker build, Docker will read this file and exclude any matching patterns.

For example, using *.jpg would exclude all JPG files. Comments can also be added using the # symbol to explain exclusions.

Utilizing a .dockerignore file helps in:

• Protecting sensitive information.
• Reducing image size by excluding unnecessary files.
• Minimizing build cache invalidation.

Inspecting Image Size

To check the size of your Docker images and containers from the command line, you can use several commands:

• docker container ls -s provides the size of running containers.
• docker image ls displays the sizes of your images.
• Use docker image history my_image:my_tag to see the sizes of intermediate images.
• docker image inspect my_image:tag reveals detailed information about your image, including layer sizes.

For deeper insights into your layers, consider using the dive package.

Best Practices for Slimming Down Docker Images

1. Utilize official base images for enhanced security and regular updates.
2. Opt for Alpine variations to maintain lightweight images.
3. Combine RUN apt-get update with apt-get install in a single instruction, chaining multiple packages efficiently.
4. Include && rm -rf /var/lib/apt/lists/* at the end of the RUN instruction to clear the apt cache.
5. Strategically position instructions likely to change lower in your Dockerfile.
6. Implement a .dockerignore file to exclude unwanted files.
7. Explore the dive tool for image layer inspection.
8. Avoid installing unnecessary packages.

Conclusion: Embrace Healthy Docker Practices

Now you're equipped to create Docker images that build and download quickly while occupying minimal space. Like maintaining a healthy diet, knowledge is half the battle. Enjoy those virtual veggies! 🥗

In the next article of this series, we'll dive into essential Docker commands. Make sure to follow along so you don’t miss out!

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Happy Dockering! 🛥