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Reducing the Container Image Size

It is often desireable to keep container images as small as possible. This can be somewhat challenging given that the union filesystem used by Docker containers is purely additive in nature. Once one or more files are added to the filesystem with the COPY or ADD build commands, subsequent commands to remove the file(s) will not reduce the container size, even though the file is deleted from the perspective of the container's filesystem.

Using the ubuntu1404_client8 example, we add the installer to the container using

COPY DeadlineClient-8.*-linux-x64-installer.run /thinkboxsetup/

a subsequent command to remove the installer after it has been run, such as

RUN rm -rf /thinkboxsetup

will not reduce the size of the built image. And given that the installer weighs in at nearly 200MB, that is rather substantial.

There are a couple of approaches to preventing the installer from adding unnecessary weight to the resulting image. One approach is to use post-build tools such as docker-slim, docker-squash, and others. These are useful tools worth learning if you make heavy use of containers, but they can introduce challenges of their own.

Here is an alternative approach that prevents the installer from remaining with the resulting image. The approach is to add the installer, run it, and remove it all in one build step. The question is, how do we get the installer into the image without using build commands like COPY or ADD?

One way is to place the installer on an internal server such that it can be retrieved with wget. You may need to add the wget program to your image first.

RUN apt-get update && apt-get -y install wget

RUN mkdir /thinkboxsetup/ \ 
    && cd /thinkboxsetup \
    && wget http://my.internal.server/deadline7_installer/DeadlineClient-7.2.3.0-linux-x64-installer.run \
    && chmod +x *.run \
    && /thinkboxsetup/DeadlineClient-7.*-linux-x64-installer.run \
        --mode unattended \
        --unattendedmodeui minimal \ 
        --repositorydir /mnt/DeadlineRepository7 \ 
        --licenseserver @lic_thinkbox \ 
        --noguimode true \ 
        --restartstalled true \
   && cd .. \
   && rm -rf /thinkboxsetup

Since the /thinkboxsetup folder is created, the installer is pulled down, the installer is executed, and the folder is removed all in one build step, the installer folder and .run file do not become part of the resulting build layer, thus preventing the hefty installer file from contributing to the final size of the image.

Note that since HTTP retrieval does not support globbing, it is necessary to specify the exact name of the installer file to be pulled down from the server. wget provides advanced options to work around the lack of globbing, but those options are beyond the scope of this document. (But here is a handy set of examples).

End