Docker Export
The export takes a snapshot of the container file system and export it’s
as a tarball, all container layers are flattened. Any metadata about the
entry point is lost, such as ENTRYPOINT and CMD.
You can run docker import to create a Docker image from that tarball.
Docker Save
Creates a tarball on the image and not the container so all the
layers are saved, and any metadata around it such as ENTRYPOINT.
You can run docker load to create a Docker image from that tarball.
When to use what
If you only care about the files that are inside of the running
container you should use docker export since it only takes the
snapshot of the container. If you care about everything about the image,
you should use docker save.
Reference
- Learned when debugging problems in https://gitlab.com/gitlab-org/gitlab-runner/-/merge_requests/2058#note_388341301
- https://github.com/moby/moby/issues/8334#issuecomment-57471427