Platform & DevOps·Packaging: containers and images
the container writes a file and it lands in a thin writable layer on top, leaving the image underneath untouched.
Overlay filesystem
Also calledoverlayfs, union filesystem, copy-on-write layer
The union mount that stacks read-only image layers and one writable layer into a single filesystem. It is why starting a container does not copy the image, and why a container that writes heavily to its own filesystem gets slow copy-on-write behaviour for files that came from a layer. Anything written there dies with the container, which is the mechanism behind logs and uploads mysteriously vanishing on restart.