different images have similar files

Rails container image and Django container image, each represented by a box with a smiley face: we both use Ubuntu 18.0!

reusing layers saves disk space

Rails image - Rails app - ubuntu:18.04

Django image - Django app - ubuntu:18.04

Both have the exact same files on disk for ubuntu:18.04.

a layer is a directory

$ ls 8891378eb*
bin/ home/ mnt/ run/ tmp/
boot/ lib/ opt sbin/ usr/
dev/ lib64/ proc/ srv/ var/
etc/ media/ root/ sys/

etc are files in an ubuntu:18.04 layer

every layer has an ID

usually the ID is a sha256 hash of the layer’s contents

example: 8e99fae2..

if a file is in 2 layers, you’ll see the version from the top layer

Two rectangular boxes on top of one another, each labelled /code/file.py. The one on top is the version you’ll see in the merged image.

by default, writes go to a temporary layer

Illustration of a rectangle labelled “temp layer”, with a bunch of other smaller rectangles stacked underneath it. The temp layer is labelled “thse files might be deleted after the container exits.”

To keep your changes, write to a directory that’s mounted from outside the container