Apply custom metadata

You can apply metadata to your images, containers, or daemons via labels. Metadata can serve a wide range of uses. Use labels to add notes or licensing information to an image or to identify a host.

A label is a <key> / <value> pair. Docker stores the label values as strings. You can specify multiple labels but each <key> / <value> must be unique to avoid overwriting. If you specify the same key several times but with different values, newer labels overwrite previous labels. Docker uses the last key=value you supply.

Note: Support for daemon-labels was added in Docker 1.4.1. Labels on containers and images are new in Docker 1.6.0

Label keys (namespaces)

Docker puts no hard restrictions on the label key you. However, labels with simple keys can conflict. For example, you can categorize your images by using a chip “architecture” label:

LABEL architecture="amd64"

LABEL architecture="ARMv7"

But a user can label images by building architectural style:

LABEL architecture="Art Nouveau"

To prevent naming conflicts, Docker namespaces label keys using a reverse domain notation. Use the following guidelines to name your keys:

All (third-party) tools should prefix their keys with the reverse DNS notation of a domain controlled by the author. For example, com.example.some-label.
The com.docker.*, io.docker.* and org.dockerproject.* namespaces are reserved for Docker's internal use.
Keys should only consist of lower-cased alphanumeric characters, dots and dashes (for example, [a-z0-9-.])
Keys should start and end with an alpha numeric character
Keys may not contain consecutive dots or dashes.
Keys without namespace (dots) are reserved for CLI use. This allows end- users to add metadata to their containers and images without having to type cumbersome namespaces on the command-line.

These are guidelines and Docker does not enforce them. Failing following these guidelines can result in conflicting labels. If you're building a tool that uses labels, you should use namespaces for your label keys.

Store structured data in labels

Label values can contain any data type that can be stored as a string. For example, consider this JSON:

{
    "Description": "A containerized foobar",
    "Usage": "docker run --rm example/foobar [args]",
    "License": "GPL",
    "Version": "0.0.1-beta",
    "aBoolean": true,
    "aNumber" : 0.01234,
    "aNestedArray": ["a", "b", "c"]
}

You can store this struct in a label by serializing it to a string first:

LABEL com.example.image-specs="{\"Description\":\"A containerized foobar\",\"Usage\":\"docker run --rm example\\/foobar [args]\",\"License\":\"GPL\",\"Version\":\"0.0.1-beta\",\"aBoolean\":true,\"aNumber\":0.01234,\"aNestedArray\":[\"a\",\"b\",\"c\"]}"

While it is possible to store structured data in label values, Docker treats this data as a ‘regular’ string. This means that Docker doesn't offer ways to query (filter) based on nested properties. If your tool needs to filter on nested properties, the tool itself should implement this.

Add labels to images; the `LABEL` instruction

Adding labels to an image:

LABEL [<namespace>.]<key>[=<value>] ...

The LABEL instruction adds a label to your image, optionally setting its value. Use surrounding quotes or backslashes for labels that contain white space character:

LABEL vendor=ACME\ Incorporated
LABEL com.example.version.is-beta
LABEL com.example.version="0.0.1-beta"
LABEL com.example.release-date="2015-02-12"

The LABEL instruction supports setting multiple labels in a single instruction using this notation:

LABEL com.example.version="0.0.1-beta" com.example.release-date="2015-02-12"

Wrapping is allowed by using a backslash (\) as continuation marker:

LABEL vendor=ACME\ Incorporated \
      com.example.is-beta \
      com.example.version="0.0.1-beta" \
      com.example.release-date="2015-02-12"

Docker recommends you add multiple labels in a single LABEL instruction. Using individual instructions for each label can result in an inefficient image. This is because each LABEL instruction in a Dockerfile produces a new IMAGE layer.

You can view the labels via the docker inspect command:

$ docker inspect 4fa6e0f0c678

...
"Labels": {
    "vendor": "ACME Incorporated",
    "com.example.is-beta": "",
    "com.example.version": "0.0.1-beta",
    "com.example.release-date": "2015-02-12"
}
...

# Inspect labels on container
$ docker inspect -f "{{json .Config.Labels }}" 4fa6e0f0c678

{"Vendor":"ACME Incorporated","com.example.is-beta":"","com.example.version":"0.0.1-beta","com.example.release-date":"2015-02-12"}

# Inspect labels on images
$ docker inspect -f "{{json .ContainerConfig.Labels }}" myimage

Query labels

Besides storing metadata, you can filter images and containers by label. To list all running containers that the com.example.is-beta label:

# List all running containers that have a `com.example.is-beta` label
$ docker ps --filter "label=com.example.is-beta"

List all running containers with a color label of blue:

$ docker ps --filter "label=color=blue"

List all images with vendor ACME:

$ docker images --filter "label=vendor=ACME"

Daemon labels

docker daemon \
  --dns 8.8.8.8 \
  --dns 8.8.4.4 \
  -H unix:///var/run/docker.sock \
  --label com.example.environment="production" \
  --label com.example.storage="ssd"

These labels appear as part of the docker info output for the daemon: