How to shrink Docker images by 90%

Recently I stumbled across a tool to reduce the size of docker images: docker-slim. Today I gave it a try and want to share my experience.

Preliminaries

My machine runs on OS X and I have Docker and Homebrew installed. Personally I prefer httpie over curl to test HTTP applications, but you can use whatever you like most.

Nodejs and Hello World

The following Dockerfile starts a container listening on port 5050.

FROM node:latest
EXPOSE 5050
CMD node --eval ' \
require("http") \
  .createServer( \
    (req,res) => res.end("Hello World!")) \
  .listen(5050); \
console.log("listing on port 5050"); \
'

Let’s run it and test it.

# build the image
docker build -t test .
 
# start a container
docker run --rm -p 5050:5050 test
 
# GET http://localhost:5050
http :5050
# > Hello World!
 
# shutdown container
docker stop …
 
# inspect local docker images
docker image list
# > test … 998MB

The Hello-World container works as expected but requires 998MB of disk space. Let’s see how much docker-slim can save us.

Optimize image with docker-slim

# download docker-slim
brew install docker-slim
 
# optimize hello world image
docker-slim build --target test 
# start a container
docker run --rm -p 5050:5050 test.slim
 
 
# GET http://localhost:5050
http :5050
# > Hello World!
 
# shutdown container
docker stop …
 
# inspect local docker images
docker image list
# > test.slim … 92.2MB

This seems nice: the image size is reduced by over 900MB to 92.2MB. This not only saves disk space but network as well, since we tend to ship our Docker images somewhere.

I hope you find this blog post helpful. If you have any feedback, feel free to contact us.