[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.