Reducing docker image size without Dockerfile

Question

I built an image a few weeks ago without using a Dockerfile using this tutorial.

Basically, you run a container from an image, edit this container and then save it as your new image, exit, commit and push. At first my image was somewhat acceptable in size but now it grows in ways i don't understand.

For example, if i need to edit my image, I will :

• docker run --name mycontainer -it from/currentimage bash
• Edit a file or configure something
• exit
• docker commit from/currentimage
• docker push from/currentimage

The problem is that changing a line in a file make the image get bigger by ~250MB. What am I doing wrong?

From there, how can I reduce the size of those, from what I think, useless layers? The other answers on SO always use a Dockerfile or reconfigure it. I don't have one in my case.

Thanks

Answer 1

I had the same issue.

Post I found, application was creating a log file (.core in my case) with large space occupied.

Post you are done with the required changes in container and before committing it, always check for high size files with below command

du -ah / | sort -rh | head -n 10

I deleted it and committed the container. Size was under control.

Answer 2

I cannot tell you why your image grows faster than you expect. You can use the export/import mechanism to flatten the layers though, which may reduce the image size:

docker create --name foo from/currentimage
docker export foo | docker import - from/currentimage:flat
docker rm foo

docker inspect from/currentimage:flat should now show a single layer.