This fix tries to address the issue raised in #22358 where syslog's
message tag always starts with `docker/` and can not be removed
by changing the log tag templates.
The issue is that syslog driver hardcodes `path.Base(os.Args[0])`
as the prefix, which is the binary file name of the daemon (`dockerd`).
This could be an issue for certain situations (e.g., #22358) where
user may prefer not to have a dedicated prefix in syslog messages.
There is no way to override this behavior in the current verison of
the docker.
This fix tries to address this issue without making changes in the
default behavior of the syslog driver. An additional
`{{.DaemonName}}` has been introduced in the syslog tag. This is
assigned as the `docker` when daemon starts. The default log tag
template has also been changed from
`path.Base(os.Args[0]) + "/{{.ID}}"` to `{{.DaemonName}}/{{.ID}}`.
Therefore, there is no behavior changes when log-tag is not provided.
In order to be consistent, the default log tag for fluentd has been
changed from `docker.{{.ID}}` to `{{DaemonName}}.{{.ID}}` as well.
The documentation for log-tag has been updated to reflect this change.
Additional test cases have been added to cover changes in this fix.
This fix fixes#22358.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
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