CONTAINERS-TRANSPORTS(5)

Valentin Rothberg April 2019

NAME
containers-transports - description of supported transports for copying and storing container images

DESCRIPTION
Tools which use the containers/image library, including skopeo(1), buildah(1), podman(1), all share a common syntax for referring to container images in various locations. The general form of the syntax is transport:details, where details are dependent on the specified transport, which are documented below.

The semantics of the image names ultimately depend on the environment where they are evaluated. For example: if evaluated on a remote server, image names might refer to paths on that server; relative paths are relative to the current directory of the image consumer.

  containers-storage:[[storage-specifier]]{image-id|docker-reference[@image-id]}
An image located in a local containers storage. The format of docker-reference is described in detail in the docker transport.

The storage-specifier allows for referencing storage locations on the file system and has the format [[driver@]root[+run-root][:options]] where the optional driver refers to the storage driver (e.g., overlay or btrfs) and where root is an absolute path to the storage's root directory. The optional run-root can be used to specify the run directory of the storage where all temporary writable content is stored. The optional options are a comma-separated list of driver-specific options. Please refer to containers-storage.conf(5) for further information on the drivers and supported options.

  dir:path
An existing local directory path storing the manifest, layer tarballs and signatures as individual files. This is a non-standardized format, primarily useful for debugging or noninvasive container inspection.

  docker://docker-reference
An image in a registry implementing the "Docker Registry HTTP API V2". By default, uses the authorization state in $XDG_RUNTIME_DIR/containers/auth.json, which is set using podman-login(1). If the authorization state is not found there, $HOME/.docker/config.json is checked, which is set using docker-login(1). The containers-registries.conf(5) further allows for configuring various settings of a registry.

Note that a docker-reference has the following format: name[:tag | @digest]. While the docker transport does not support both a tag and a digest at the same time some formats like containers-storage do. Digests can also be used in an image destination as long as the manifest matches the provided digest.

The docker transport supports pushing images without a tag or digest to a registry when the image name is suffixed with @@unknown-digest@@. The name@@unknown-digest@@ reference format cannot be used with a reference that has a tag or digest. The digest of images can be explored with skopeo-inspect(1).

If name does not contain a slash, it is treated as docker.io/library/name. Otherwise, the component before the first slash is checked if it is recognized as a hostname ⟨i.e., it contains either a `.` or a `:`, or the component is exactly `localhost`⟩. If the first component of name is not recognized as a hostname[:port], name is treated as docker.io/name.

  docker-archive:path[:{docker-reference|@source-index}]
An image is stored in the docker-save(1) formatted file.

Unless a tool explicitly documents otherwise, a write to a docker-archive: destination completely overwrites path, replacing it with the single provided image.

The path can refer to a stream, e.g. docker-archive:/dev/stdin.

docker-reference must not contain a digest. Alternatively, for reading archives, @source-index is a zero-based index in archive manifest (to access untagged images). If neither docker-reference nor @_source_index is specified when reading an archive, the archive must contain exactly one image.

  docker-daemon:docker-reference|algo:digest
An image stored in the docker daemon's internal storage. The image must be specified as a docker-reference or in an alternative algo:digest format when being used as an image source. The algo:digest refers to the image ID reported by docker-inspect(1).

  oci:path[:{reference|@source-index}]
An image in a directory structure compliant with the "Open Container Image Layout Specification" at path.

The path value terminates at the first : character; any further : characters are not separators, but a part of reference. The reference is used to set, or match, the org.opencontainers.image.ref.name annotation in the top-level index. For reading images, @source-index is a zero-based index in manifest (to access untagged images). If neither reference nor @_source_index is specified when reading an image, the path must contain exactly one image.

  oci-archive:path[:reference]
An image in a tar(1) archive with contents compliant with the "Open Container Image Layout Specification" at path.

Unless a tool explicitly documents otherwise, a write to an oci-archive: destination completely overwrites path, replacing it with the single provided image.

The path value terminates at the first : character; any further : characters are not separators, but a part of reference. The reference is used to set, or match, the org.opencontainers.image.ref.name annotation in the top-level index. If reference is not specified when reading an archive, the archive must contain exactly one image.

  sif:path
An image using the Singularity image format at path.

Only reading images is supported, and not all scripts can be represented in the OCI format.

Examples
The following examples demonstrate how some of the containers transports can be used. The examples use skopeo-copy(1) for copying container images.

Copying an image from one registry to another:
    $ skopeo copy docker://docker.io/library/alpine:latest docker://localhost:5000/alpine:latest

Copying an image from a running Docker daemon to a directory in the OCI layout:
    $ mkdir alpine-oci
    $ skopeo copy docker-daemon:alpine:latest oci:alpine-oci
    $ tree alpine-oci
    test-oci/
    ├── blobs
    │   └── sha256
    │       ├── 83ef92b73cf4595aa7fe214ec6747228283d585f373d8f6bc08d66bebab531b7
    │       ├── 9a6259e911dcd0a53535a25a9760ad8f2eded3528e0ad5604c4488624795cecc
    │       └── ff8df268d29ccbe81cdf0a173076dcfbbea4bb2b6df1dd26766a73cb7b4ae6f7
    ├── index.json
    └── oci-layout
    2 directories, 5 files

Copying an image from a registry to the local storage:
    $ skopeo copy docker://docker.io/library/alpine:latest containers-storage:alpine:latest

SEE ALSO
docker-login(1), docker-save(1), podman-login(1), skopeo-copy(1), skopeo-inspect(1), tar(1), container-registries.conf(5), containers-storage.conf(5)

AUTHORS
Miloslav Trmač mitr@redhat.com ⟨mailto:mitr@redhat.com⟩ Valentin Rothberg rothberg@redhat.com ⟨mailto:rothberg@redhat.com⟩