How to Create a Tiny Docker Image for your Ruby App

Travis Reeder
Sep 8, 2015 · 2 min read
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I’ve been on a quest to create the smallest possible Docker images for all the things, and here’s how to create a very small image for your Ruby apps (much smaller than I would have thought possible for Ruby). This post will walk you through packaging up a simple Sinatra app into a Docker image that weighs in around 18MB.

All you need for this tutorial is Docker, you don’t even need Ruby installed.

0) The App

That is a Sinatra app that will return a jsonified Hello World response. We need a Gemfile to define our dependencies too:

1) Vendor Dependencies

The iron/ruby-bundle image we’re using here is a special image that has all the libs we need to build native extensions.

2) Test the App

Check http://localhost:8080/ to ensure it’s running correctly. Notice we’re using iron/ruby here, this is a much smaller image than iron/ruby-bundle and has everything we need to run the app. Both of these iron/ruby* images are based on the very small Alpine Linux image (which is totally awesome).

3) Build Docker Image

Now build the image:

4) Test the Docker Image

Once again, check http://localhost:8080/ to ensure it’s running correctly.

Conclusion

You can find the full source code for this example here: https://github.com/treeder/hello-sinatra

One last thing, if you want to distribute your app, just push it up to Docker Hub:

Then anyone can run your app just by running the same docker run command as above.

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