Refs #10754 apply partial fix to Dockerfile which is from Commit d9d69bb5....

Refs #10754 apply partial fix to Dockerfile which is from Commit d9d69bb5. Commit d9d69bb5 was rejected due to  "rundumschlag", see ticket.

Documentation/docker-sphinx-qfq/Dockerfile

@@ -5,4 +5,5 @@ ADD requirements.txt /docs
 RUN apt-get update && \
     apt-get upgrade -y && \
     apt-get install -y git
-RUN pip3 install -r requirements.txt
\ No newline at end of file
+RUN pip3 install -r requirements.txt
+RUN rm requirements.txt