Fix expected docker file. (apache-spark-on-k8s#401)

The generated file is correct but the expected file in the test was not.

Author: mccheah

spark-docker-image-generator/src/test/resources/ExpectedDockerfile

@@ -31,7 +31,7 @@ RUN set -ex && \
     apk upgrade --no-cache && \
     apk add --no-cache bash tini && \
     mkdir -p /opt/spark && \
-    mkdir -p /opt/spark/work-dir \
+    mkdir -p /opt/spark/work-dir && \
     touch /opt/spark/RELEASE && \
     rm /bin/sh && \
     ln -sv /bin/bash /bin/sh && \