Summarize some methods to slim down docker images

Docker is quickly becoming synonymous with application containerization solutions. However, the size of Docker images often becomes one of the dilemmas that are easy to fall into when using Docker. It’s obvious that while many Docker images contain a lot of useful components, they often also contain a lot of unnecessary elements that take up a lot of hard drive space. This can become a problem, especially when you need to deploy a large number of Docker images simultaneously. How to minimize the size of these Docker images? This article will introduce some methods that can help you slim down your Docker image.

1. Select the smaller image as the base image.

When you build a Docker image, the base image is very important. They contain the main components of a Docker image, so choosing the right base image can significantly reduce the size of your Docker image. In order to slim down your Docker image, you should try to choose a smaller base image.

For example, if you are writing a Python application, it is better to use Alpine Linux than Ubuntu because Alpine Linux only contains the necessary packages and libraries, so its size is much smaller than Ubuntu.

2. Combine multiple RUN instructions into one instruction.

Each RUN instruction in the Dockerfile creates a new image layer in the container. This means that every RUN instruction you use increases the size of the image. Therefore, it is very important to combine multiple commands into a single RUN instruction whenever possible.

Here is an example:

# 不推荐的写法：
RUN apt-get update && apt-get install -y package1 package2 \
    && apt-get clean && rm -rf /var/lib/apt/lists/*

# 推荐的写法：
RUN apt-get update \
    && apt-get install -y package1 package2 \
    && apt-get clean \
    && rm -rf /var/lib/apt/lists/*

3. Delete useless files.

When building a Docker image, many unnecessary files may be included. This may include caches of packages or other intermediately generated files. These files take up a lot of space in the image, so removing them can significantly reduce the size of the image.

Here are some files that may take up space:

• /var/cache/apt/archives
• /usr/share/doc
• /usr/share/man
• /usr/share/info
• /usr/share/locale
• /usr/share/gtk-doc

You can use the following command to remove them:

RUN apt-get update \
    && apt-get install -y package1 package2 \
    && apt-get clean \
    && rm -rf /var/lib/apt/lists/* \
    && rm -rf /var/cache/apt/archives/* \
    && rm -rf /usr/share/doc/* \
    && rm -rf /usr/share/man/* \
    && rm -rf /usr/share/info/* \
    && rm -rf /usr/share/locale/* \
    && rm -rf /usr/share/gtk-doc/*

4. Use smaller alternatives.

Sometimes, you may use a library or tool, but you don't actually need all its features. In this case, using a lightweight alternative is a good way to reduce the image size.

For example, if you need to run Java applications in containers, Oracle JDK may be larger than OpenJDK. In this case it might be better to use OpenJDK as it is smaller than Oracle JDK.

5. Compress/Decompress.

When packaging an application into a Docker image, compress the application files so that they use less space in the image. When the Docker container is running, use the "docker run" command, and these files will be automatically decompressed inside the container.

The following is an example:

# 将文件压缩并将其移动到容器中。
tar -czvf myapp.tar.gz /path/to/myapp
mv myapp.tar.gz /docker/myapp/

# 在Dockerfile中使用ADD指令将文件解压并复制到容器中。
ADD myapp.tar.gz /app/

In summary, by selecting a smaller base image, merging multiple RUN instructions into one instruction, deleting useless files, and using a smaller Replacements, as well as compressing/decompressing images, can significantly reduce the size of Docker images. Through these methods, you can ensure that your Docker image only contains necessary files and components, thereby reducing the size and complexity of container deployment and scaling.

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