Docs builder fixes (#7815)

* Add missing command separator

* Use Python 3.8 to build the documentation

infrastructure/docker/build/Dockerfile-docs

@@ -35,7 +35,7 @@ RUN rpm --import /etc/pki/rpm-gpg/RPM-GPG-KEY-* && \
 		rpm-build \
 		rsync \
 		epel-release && \
-	yum -y clean all \
+	yum -y clean all && \
 	# Add skip git's safe.directory check
 	git config --global --add safe.directory '*'
 
@@ -51,8 +51,8 @@ RUN if [[ ${RHEL_VERSION%%.*} -ge 8 ]]; then \
 ADD docs/source/requirements.txt /docs.requirements.txt
 RUN	yum -y install \
 		pandoc \
-		python3 \
-		python3-pip \
+		python38 \
+		python38-pip \
 		make && \
 	yum -y clean all
 RUN	python3 -m pip install --upgrade setuptools && \