An Image is an ordered collection of root filesystem changes and the corresponding execution parameters for use within a container runtime. This specification outlines the format of these filesystem changes and corresponding parameters and describes how to create and use them for use with a container runtime and execution tool.
This version of the image specification was adopted starting in Docker 1.12.
This specification uses the following terms:
Here is an example image JSON file:
{ "created": "2015-10-31T22:22:56.015925234Z", "author": "Alyssa P. Hacker <alyspdev@example.com>", "architecture": "amd64", "os": "linux", "config": { "User": "alice", "Memory": 2048, "MemorySwap": 4096, "CpuShares": 8, "ExposedPorts": { "8080/tcp": {} }, "Env": [ "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin", "FOO=docker_is_a_really", "BAR=great_tool_you_know" ], "Entrypoint": [ "/bin/my-app-binary" ], "Cmd": [ "--foreground", "--config", "/etc/my-app.d/default.cfg" ], "Volumes": { "/var/job-result-data": {}, "/var/log/my-app-logs": {}, }, "WorkingDir": "/home/alice", }, "rootfs": { "diff_ids": [ "sha256:c6f988f4874bb0add23a778f753c65efe992244e148a1d2ec2a8b664fb66bbd1", "sha256:5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef" ], "type": "layers" }, "history": [ { "created": "2015-10-31T22:22:54.690851953Z", "created_by": "/bin/sh -c #(nop) ADD file:a3bc1e842b69636f9df5256c49c5374fb4eef1e281fe3f282c65fb853ee171c5 in /" }, { "created": "2015-10-31T22:22:55.613815829Z", "created_by": "/bin/sh -c #(nop) CMD [\"sh\"]", "empty_layer": true } ] }
Note that image JSON files produced by Docker don't contain formatting whitespace. It has been added to this example for clarity.
<h4>Container RunConfig Field Descriptions</h4> <dl> <dt> User <code>string</code> </dt> <dd> <p>The username or UID which the process in the container should run as. This acts as a default value to use when the value is not specified when creating a container.</p> <p>All of the following are valid:</p> <ul> <li><code>user</code></li> <li><code>uid</code></li> <li><code>user:group</code></li> <li><code>uid:gid</code></li> <li><code>uid:group</code></li> <li><code>user:gid</code></li> </ul> <p>If <code>group</code>/<code>gid</code> is not specified, the default group and supplementary groups of the given <code>user</code>/<code>uid</code> in <code>/etc/passwd</code> from the container are applied.</p> </dd> <dt> Memory <code>integer</code> </dt> <dd> Memory limit (in bytes). This acts as a default value to use when the value is not specified when creating a container. </dd> <dt> MemorySwap <code>integer</code> </dt> <dd> Total memory usage (memory + swap); set to <code>-1</code> to disable swap. This acts as a default value to use when the value is not specified when creating a container. </dd> <dt> CpuShares <code>integer</code> </dt> <dd> CPU shares (relative weight vs. other containers). This acts as a default value to use when the value is not specified when creating a container. </dd> <dt> ExposedPorts <code>struct</code> </dt> <dd> A set of ports to expose from a container running this image. This JSON structure value is unusual because it is a direct JSON serialization of the Go type <code>map[string]struct{}</code> and is represented in JSON as an object mapping its keys to an empty object. Here is an example:
Its keys can be in the format of: <ul> <li> <code>"port/tcp"</code> </li> <li> <code>"port/udp"</code> </li> <li> <code>"port"</code> </li> </ul> with the default protocol being <code>"tcp"</code> if not specified. These values act as defaults and are merged with any specified when creating a container. </dd> <dt> Env <code>array of strings</code> </dt> <dd> Entries are in the format of <code>VARNAME="var value"</code>. These values act as defaults and are merged with any specified when creating a container. </dd> <dt> Entrypoint <code>array of strings</code> </dt> <dd> A list of arguments to use as the command to execute when the container starts. This value acts as a default and is replaced by an entrypoint specified when creating a container. </dd> <dt> Cmd <code>array of strings</code> </dt> <dd> Default arguments to the entry point of the container. These values act as defaults and are replaced with any specified when creating a container. If an <code>Entrypoint</code> value is not specified, then the first entry of the <code>Cmd</code> array should be interpreted as the executable to run. </dd> <dt> Healthcheck <code>struct</code> </dt> <dd> A test to perform to determine whether the container is healthy. Here is an example:
The object has the following fields. <dl> <dt> Test <code>array of strings</code> </dt> <dd> The test to perform to check that the container is healthy. The options are: <ul> <li><code>[]</code> : inherit healthcheck from base image</li> <li><code>["NONE"]</code> : disable healthcheck</li> <li><code>["CMD", arg1, arg2, ...]</code> : exec arguments directly</li> <li><code>["CMD-SHELL", command]</code> : run command with system's default shell</li> </ul> The test command should exit with a status of 0 if the container is healthy, or with 1 if it is unhealthy. </dd> <dt> Interval <code>integer</code> </dt> <dd> Number of nanoseconds to wait between probe attempts. </dd> <dt> Timeout <code>integer</code> </dt> <dd> Number of nanoseconds to wait before considering the check to have hung. </dd> <dt> Retries <code>integer</code> <dt> <dd> The number of consecutive failures needed to consider a container as unhealthy. </dd> </dl> In each case, the field can be omitted to indicate that the value should be inherited from the base layer. These values act as defaults and are merged with any specified when creating a container. </dd> <dt> Volumes <code>struct</code> </dt> <dd> A set of directories which should be created as data volumes in a container running this image. This JSON structure value is unusual because it is a direct JSON serialization of the Go type <code>map[string]struct{}</code> and is represented in JSON as an object mapping its keys to an empty object. Here is an example:
</dd> <dt> WorkingDir <code>string</code> </dt> <dd> Sets the current working directory of the entry point process in the container. This value acts as a default and is replaced by a working directory specified when creating a container. </dd> </dl> </dd> <dt> rootfs <code>struct</code> </dt> <dd> The rootfs key references the layer content addresses used by the image. This makes the image config hash depend on the filesystem hash. rootfs has two subkeys: <ul> <li> <code>type</code> is usually set to <code>layers</code>. </li> <li> <code>diff_ids</code> is an array of layer content hashes (<code>DiffIDs</code>), in order from bottom-most to top-most. </li> </ul> Here is an example rootfs section:
</dd> <dt> history <code>struct</code> </dt> <dd> <code>history</code> is an array of objects describing the history of each layer. The array is ordered from bottom-most layer to top-most layer. The object has the following fields. <ul> <li> <code>created</code>: Creation time, expressed as a ISO-8601 formatted combined date and time </li> <li> <code>author</code>: The author of the build point </li> <li> <code>created_by</code>: The command which created the layer </li> <li> <code>comment</code>: A custom message set when creating the layer </li> <li> <code>empty_layer</code>: This field is used to mark if the history item created a filesystem diff. It is set to true if this history item doesn't correspond to an actual layer in the rootfs section (for example, a command like ENV which results in no change to the filesystem). </li> </ul>
Here is an example history section:
</dd>
Any extra fields in the Image JSON struct are considered implementation specific and should be ignored by any implementations which are unable to interpret them.
An example of creating an Image Filesystem Changeset follows.
An image root filesystem is first created as an empty directory. Here is the initial empty directory structure for the a changeset using the randomly-generated directory name c3167915dc9d
(actual layer DiffIDs are generated based on the content).
c3167915dc9d/
Files and directories are then created:
c3167915dc9d/ etc/ my-app-config bin/ my-app-binary my-app-tools
The c3167915dc9d
directory is then committed as a plain Tar archive with entries for the following files:
etc/my-app-config bin/my-app-binary bin/my-app-tools
To make changes to the filesystem of this container image, create a new directory, such as f60c56784b83
, and initialize it with a snapshot of the parent image's root filesystem, so that the directory is identical to that of c3167915dc9d
. NOTE: a copy-on-write or union filesystem can make this very efficient:
f60c56784b83/ etc/ my-app-config bin/ my-app-binary my-app-tools
This example change is going add a configuration directory at /etc/my-app.d
which contains a default config file. There's also a change to the my-app-tools
binary to handle the config layout change. The f60c56784b83
directory then looks like this:
f60c56784b83/ etc/ my-app.d/ default.cfg bin/ my-app-binary my-app-tools
This reflects the removal of /etc/my-app-config
and creation of a file and directory at /etc/my-app.d/default.cfg
. /bin/my-app-tools
has also been replaced with an updated version. Before committing this directory to a changeset, because it has a parent image, it is first compared with the directory tree of the parent snapshot, f60c56784b83
, looking for files and directories that have been added, modified, or removed. The following changeset is found:
Added: /etc/my-app.d/default.cfg Modified: /bin/my-app-tools Deleted: /etc/my-app-config
A Tar Archive is then created which contains only this changeset: The added and modified files and directories in their entirety, and for each deleted item an entry for an empty file at the same location but with the basename of the deleted file or directory prefixed with .wh.
. The filenames prefixed with .wh.
are known as “whiteout” files. NOTE: For this reason, it is not possible to create an image root filesystem which contains a file or directory with a name beginning with .wh.
. The resulting Tar archive for f60c56784b83
has the following entries:
/etc/my-app.d/default.cfg /bin/my-app-tools /etc/.wh.my-app-config
Any given image is likely to be composed of several of these Image Filesystem Changeset tar archives.
There is also a format for a single archive which contains complete information about an image, including:
For example, here's what the full archive of library/busybox
is (displayed in tree
format):
. ├── 47bcc53f74dc94b1920f0b34f6036096526296767650f223433fe65c35f149eb.json ├── 5f29f704785248ddb9d06b90a11b5ea36c534865e9035e4022bb2e71d4ecbb9a │ ├── VERSION │ ├── json │ └── layer.tar ├── a65da33792c5187473faa80fa3e1b975acba06712852d1dea860692ccddf3198 │ ├── VERSION │ ├── json │ └── layer.tar ├── manifest.json └── repositories
There is a directory for each layer in the image. Each directory is named with a 64 character hex name that is deterministically generated from the layer information. These names are not necessarily layer DiffIDs or ChainIDs. Each of these directories contains 3 files:
VERSION
- The schema version of the json
filejson
- The legacy JSON metadata for an image layer. In this version of the image specification, layers don't have JSON metadata, but in version 1, they did. A file is created for each layer in the v1 format for backward compatibility.layer.tar
- The Tar archive of the filesystem changeset for an image layer.Note that this directory layout is only important for backward compatibility. Current implementations use the paths specified in manifest.json
.
The content of the VERSION
files is simply the semantic version of the JSON metadata schema:
1.0
The repositories
file is another JSON file which describes names/tags:
{ "busybox":{ "latest":"5f29f704785248ddb9d06b90a11b5ea36c534865e9035e4022bb2e71d4ecbb9a" } }
Every key in this object is the name of a repository, and maps to a collection of tag suffixes. Each tag maps to the ID of the image represented by that tag. This file is only used for backwards compatibility. Current implementations use the manifest.json
file instead.
The manifest.json
file provides the image JSON for the top-level image, and optionally for parent images that this image was derived from. It consists of an array of metadata entries:
[ { "Config": "47bcc53f74dc94b1920f0b34f6036096526296767650f223433fe65c35f149eb.json", "RepoTags": ["busybox:latest"], "Layers": [ "a65da33792c5187473faa80fa3e1b975acba06712852d1dea860692ccddf3198/layer.tar", "5f29f704785248ddb9d06b90a11b5ea36c534865e9035e4022bb2e71d4ecbb9a/layer.tar" ] } ]
There is an entry in the array for each image.
The Config
field references another file in the tar which includes the image JSON for this image.
The RepoTags
field lists references pointing to this image.
The Layers
field points to the filesystem changeset tars.
An optional Parent
field references the imageID of the parent image. This parent must be part of the same manifest.json
file.
This file shouldn't be confused with the distribution manifest, used to push and pull images.
Generally, implementations that support this version of the spec will use the manifest.json
file if available, and older implementations will use the legacy */json
files and repositories
.