Log tags for logging driver

The tag log option specifies how to format a tag that identifies the container’s log messages. By default, the system uses the first 12 characters of the container id. To override this behavior, specify a tag option:

$ docker run --log-driver=fluentd --log-opt fluentd-address=myhost.local:24224 --log-opt tag="mailer"

Docker supports some special template markup you can use when specifying a tag’s value:

For example, specifying a --log-opt tag="{{.ImageName}}/{{.Name}}/{{.ID}}" value yields syslog log lines like:

Aug  7 18:33:19 HOSTNAME docker/hello-world/foobar/5790672ab6a0[9103]: Hello from Docker.

At startup time, the system sets the container_name field and {{.Name}} in the tags. If you use docker rename to rename a container, the new name is not reflected in the log messages. Instead, these messages continue to use the original container name.

For advanced usage, the generated tag’s use go templates and the container’s logging context.

As an example of what is possible with the syslog logger, if you use the following command, you get the output that follows:

$ docker run -it --rm \
    --log-driver syslog \
    --log-opt tag="{{ (.ExtraAttributes nil).SOME_ENV_VAR }}" \
    --log-opt env=SOME_ENV_VAR \
    -e SOME_ENV_VAR=logtester.1234 \
    flyinprogrammer/logtester

Apr  1 15:22:17 ip-10-27-39-73 docker/logtester.1234[45499]: + exec app
Apr  1 15:22:17 ip-10-27-39-73 docker/logtester.1234[45499]: 2016-04-01 15:22:17.075416751 +0000 UTC stderr msg: 1

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