[SPARK-24530][FOLLOWUP] run Sphinx with python 3 in docker

## What changes were proposed in this pull request?

SPARK-24530 discovered a problem of generation python doc, and provided a fix: setting SPHINXPYTHON to python 3.

This PR makes this fix automatic in the release script using docker.

## How was this patch tested?

verified by the 2.4.0 rc2

Closes #22607 from cloud-fan/python.

Authored-by: Wenchen Fan <wenchen@databricks.com>
Signed-off-by: Marcelo Vanzin <vanzin@cloudera.com>

Author: cloud-fan

dev/create-release/do-release-docker.sh

@@ -135,6 +135,9 @@ if [ -n "$JAVA" ]; then
   JAVA_VOL="--volume $JAVA:/opt/spark-java"
 fi
 
+# SPARK-24530: Sphinx must work with python 3 to generate doc correctly.
+echo "SPHINXPYTHON=/opt/p35/bin/python" >> $ENVFILE
+
 echo "Building $RELEASE_TAG; output will be at $WORKDIR/output"
 docker run -ti \
   --env-file "$ENVFILE" \

dev/create-release/spark-rm/Dockerfile

@@ -62,8 +62,8 @@ RUN echo 'deb http://cran.cnr.Berkeley.edu/bin/linux/ubuntu xenial/' >> /etc/apt
   pip install $BASE_PIP_PKGS && \
   pip install $PIP_PKGS && \
   cd && \
-  virtualenv -p python3 p35 && \
-  . p35/bin/activate && \
+  virtualenv -p python3 /opt/p35 && \
+  . /opt/p35/bin/activate && \
   pip install $BASE_PIP_PKGS && \
   pip install $PIP_PKGS && \
   # Install R packages and dependencies used when building.