How does Docker keep the image immutable
docker, unionfs
Solution
If you have a file in a layer and modify it (using `RUN`, or `COPY` or `ADD`), a new layer is created with the new entire file, not delta. Even worse if you only change the permission attributes of the file, `RUN chmod 400 file` a new layer is created and the whole file content reside in this new layer.
Regards
Problem
From the Docker documentation : The Docker image is read-only. When Docker runs a container from an image, it adds a read-write layer on top of the image (using a UnionFS) in which your application runs. How are changes reconciled across layers? If I change the content of a file, would Docker only keep track of delta or will it store the altered file in the new layer? I looked at this discussion at superuser, but still not certain about the final image structure.