Writable layer

Each container gets a thin writable layer on top of its image. Every file it creates or changes lands there, and that layer is deleted with the container.

How it works

The image layers stay read-only. When a container modifies a file of the image, the file is first copied up into its writable layer (copy-on-write), so other containers of the same image keep seeing the original.

What breaks

Data written there, a database or uploaded files, is lost by docker rm, by docker compose down or when a container is recreated for an upgrade. The layer also grows silently on the server's disk. Anything that must outlive the container goes in a volume.

In a terminal

docker run -d --name web nginx:1.27
docker exec web sh -c 'echo hello > /usr/share/nginx/html/note.txt'
docker rm -f web
docker run -d --name web nginx:1.27
docker exec web cat /usr/share/nginx/html/note.txt

See it break, then fix it

Write the Dockerfile or the compose file, type the docker commands, and watch a simulated server run them through reboots, upgrades and traffic. Free to start.

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