[SPARK-20781] the location of Dockerfile in docker.properties.templat is wrong
[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 749418d285461958a0f22ed355edafd87f1ee913)
Signed-off-by: Sean Owen <291c18f3fb7528c712d9098b0e50a515ea0b91d5@cloudera.com>

jlopezmalla/spark,jlopezmalla/spark,apache-spark-on-k8s/spark,apache-spark-on-k8s/spark,apache-spark-on-k8s/spark,apache-spark-on-k8s/spark,jlopezmalla/spark,publicRoman/spark,publicRoman/spark,publicRoman/spark,jlopezmalla/spark,jlopezmalla/spark,jlopezmalla/spark,publicRoman/spark,publicRoman/spark,publicRoman/spark,publicRoman/spark,apache-spark-on-k8s/spark,jlopezmalla/spark,apache-spark-on-k8s/spark,apache-spark-on-k8s/spark