Yesterday I spent some time researching how to reduce the size of my Docker image. Weighing in at 841.6 MB while loaded my Rails application image is probably average sized. Docker Hub reports it as 338 MB which is approximately the size of the image after I have docker saved and gzip’d it. That tells me that docker push  and docker pull are transporting much lighter files than I work with on a daily basis. That’s good, but smaller images mean faster deploys, so time researching how to lighten their load seemed worthwhile.

Known techniques for reducing Docker image size

There are some decent articles that discuss Docker image size in depth so I will only summarize here. First and foremost, build from a small base image. Don’t use a full-blown Ubuntu desktop image if all you need is a minimal Linux instance that can run your application. That should be obvious. Secondly, chain your RUN commands together when appropriate. Apparently every statement in your Dockerfile produces a layer of your final image, and each layer is an image itself. Reduce the number of layers and your resulting image should be smaller. That said && should be your friend in your Dockerfile. For example, you might as well combine two statements like this:

into one:

Doing so saved me 11.1 MB. As discussed below in the comments, this is mostly useful when you chain statements that dirty your image with statements that clean up the dirt. If left in two statements then the first will create a layer with all the junk that the second statement cleans up. Chain them together and the junk won’t be in the final image at all.

A word on dependencies

I think gems are likely a big bloat culprit in any Rails app image. That said, you should question every gem in your Gemfile and make sure it’s serving a purpose in your app. Remove gems that aren’t. Same is true for any other software platform — scrutinize your dependencies, and get rid of the cruft. Not only will this reduce your Docker image size but it will help keep your application maintainable.

Known tools for reducing Docker image size

I came across two tools that claim to reduce the size of your existing Docker images. One is called docker-slim and the other docker-squash. I tried them both out, but didn’t have success with either. docker-slim did dramatically reduce my image size from 841.6 MB to an amazingly small 31.59 MB, but afterwards my Rails app failed to boot and I saw failures in my unicorn log file that looked like this:

Clearly docker-slim chewed up my application and spit it out into something unusable. On the other hand, I gave up on making docker-squash work simply because I was too lazy to install its hard dependency of tar version 1.27. My CI server is stuck at Ubuntu 12.04 (precise) and tar 1.27 isn’t easily available until 14.04. Yes, I could compile from source or try to pull the 14.04 binaries. However, at the end of the day I just didn’t want to go down those roads. In my experience any tool that forces you in those directions just isn’t ready for prime time.

Despite not having luck with docker-squash or docker-slim I applaud the authors of these tools. They are taking on a real issue in the Docker community and have to keep pace with some fast moving, cutting-edge technology. I’ll be watching these projects; I wouldn’t be surprised if one or the other matured on its own or had its ideas absorbed into Docker itself.

Got questions or feedback? I want it. Drop your thoughts in the comments below or hit me @ccstump. You can also follow me on Twitter to be aware of future articles.

Thanks for reading!