Revert "chore(python): use ubuntu 22.04 in docs image (googleapis#666)"

This reverts commit 6f384e2.

.github/.OwlBot.lock.yaml

@@ -13,5 +13,5 @@
 # limitations under the License.
 docker:
   image: gcr.io/cloud-devrel-public-resources/owlbot-python:latest
-  digest: sha256:bc5eed3804aec2f05fad42aacf973821d9500c174015341f721a984a0825b6fd
-# created: 2022-04-21T15:43:16.246106921Z
+  digest: sha256:00c9d764fd1cd56265f12a5ef4b99a0c9e87cf261018099141e2ca5158890416
+# created: 2022-04-20T23:42:53.970438194Z

.kokoro/docker/docs/Dockerfile

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from ubuntu:22.04
+from ubuntu:20.04
 
 ENV DEBIAN_FRONTEND noninteractive
 
@@ -60,24 +60,8 @@ RUN apt-get update \
   && rm -rf /var/lib/apt/lists/* \
   && rm -f /var/cache/apt/archives/*.deb
 
-###################### Install python 3.8.11
-
-# Download python 3.8.11
-RUN wget https://www.python.org/ftp/python/3.8.11/Python-3.8.11.tgz
-
-# Extract files
-RUN tar -xvf Python-3.8.11.tgz
-
-# Install python 3.8.11
-RUN ./Python-3.8.11/configure --enable-optimizations
-RUN make altinstall
-
-###################### Install pip
 RUN wget -O /tmp/get-pip.py 'https://bootstrap.pypa.io/get-pip.py' \
-  && python3 /tmp/get-pip.py \
+  && python3.8 /tmp/get-pip.py \
   && rm /tmp/get-pip.py
 
-# Test pip
-RUN python3 -m pip
-
 CMD ["python3.8"]