[SPARK-20781] the location of Dockerfile in docker.properties.templat is wrong

[https://issues.apache.org/jira/browse/SPARK-20781](https://issues.apache.org/jira/browse/SPARK-20781)


the location of Dockerfile in docker.properties.template should be "../external/docker/spark-mesos/Dockerfile"

Author: liuzhaokun <liu.zhaokun@zte.com.cn>

Closes #18013 from liu-zhaokun/dockerfile_location.

(cherry picked from commit 749418d2)
Signed-off-by: Sean Owen <sowen@cloudera.com>

conf/docker.properties.template

@@ -15,6 +15,6 @@
 # limitations under the License.
 #
 
-spark.mesos.executor.docker.image: <image built from `../docker/spark-mesos/Dockerfile`>
+spark.mesos.executor.docker.image: <image built from `../external/docker/spark-mesos/Dockerfile`>
 spark.mesos.executor.docker.volumes: /usr/local/lib:/host/usr/local/lib:ro
 spark.mesos.executor.home: /opt/spark