containerd/vendor/github.com/opencontainers/specs/bundle.md

Filesystem Bundle

Container Format

This section defines a format for encoding a container as a filesystem bundle - a set of files organized in a certain way, and containing all the necessary data and metadata for any compliant runtime to perform all standard operations against it. See also OS X application bundles for a similar use of the term bundle.

The definition of a bundle is only concerned with how a container, and its configuration data, are stored on a local file system so that it can be consumed by a compliant runtime.

A Standard Container bundle contains all the information needed to load and run a container. This includes the following artifacts which MUST all reside in the same directory on the local filesystem:

1. config.json : contains configuration data. This REQUIRED file, which MUST be named config.json. When the bundle is packaged up for distribution, this file MUST be included. See config.json for more details.

2. A directory representing the root filesystem of the container. While the name of this REQUIRED directory may be arbitrary, users should consider using a conventional name, such as rootfs. When the bundle is packaged up for distribution, this directory MUST be included. This directory MUST be referenced from within the config.json file.

While these artifacts MUST all be present in a single directory on the local filesystem, that directory itself is not part of the bundle. In other words, a tar archive of a bundle will have these artifacts at the root of the archive, not nested within a top-level directory.