Add syslog-tag option for syslog log-driver

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

daemon/logger/syslog/syslog.go

@@ -29,7 +29,10 @@ func init() {
 }
 
 func New(ctx logger.Context) (logger.Logger, error) {
-	tag := ctx.ContainerID[:12]
+	tag := ctx.Config["syslog-tag"]
+	if tag == "" {
+		tag = ctx.ContainerID[:12]
+	}
 
 	proto, address, err := parseAddress(ctx.Config["syslog-address"])
 	if err != nil {

docs/sources/reference/run.md

@@ -884,6 +884,7 @@ The following logging options are supported for this logging driver:
 
     --log-opt address=[tcp|udp]://host:port
     --log-opt address=unix://path
+    --log-opt syslog-tag="mailer"
 
 `address` specifies the remote syslog server address where the driver connects to.
 If not specified it defaults to the local unix socket of the running system.
@@ -893,6 +894,8 @@ remote server at `192.168.0.42` on port `123`
 
     $ docker run --log-driver=syslog --log-opt address=tcp://192.168.0.42:123
 
+`syslog-tag` specifies tag for syslog messages from container.
+
 #### Logging driver: journald
 
 Journald logging driver for Docker. Writes log messages to journald; the container id will be stored in the journal's `CONTAINER_ID` field. `docker logs` command is not available for this logging driver.  For detailed information on working with this logging driver, see [the journald logging driver](reference/logging/journald) reference documentation.