Revert "Update images to use python 3.8"

This breaks ensure-twine in opendev which is currently incompatible
with py38.

This reverts commit 429d73e93d.

Change-Id: I983b871d6b42b6d190bc055b57ba0a5f1a4618c0

.zuul.yaml

@@ -141,8 +141,8 @@
     allowed-projects: zuul/zuul
     timeout: 2700  # 45 minutes
     requires:
-      - python-builder-3.8-container-image
-      - python-base-3.8-container-image
+      - python-builder-3.7-container-image
+      - python-base-3.7-container-image
     provides: zuul-container-image
     vars: &zuul_image_vars
       docker_images:
@@ -180,8 +180,8 @@
     description: Build Docker images and upload to Docker Hub.
     allowed-projects: zuul/zuul
     requires:
-      - python-builder-3.8-container-image
-      - python-base-3.8-container-image
+      - python-builder-3.7-container-image
+      - python-base-3.7-container-image
     provides: zuul-container-image
     secrets:
       name: docker_credentials

Dockerfile

@@ -13,7 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM docker.io/opendevorg/python-builder:3.8 as builder
+FROM docker.io/opendevorg/python-builder:3.7 as builder
 ENV DEBIAN_FRONTEND=noninteractive
 
 # Optional location of Zuul API endpoint.
@@ -47,7 +47,7 @@ RUN mkdir /tmp/openshift-install \
   && echo $OPENSHIFT_SHA /tmp/openshift-install/openshift-client.tgz | sha256sum --check \
   && tar xvfz openshift-client.tgz --strip-components=1 -C /tmp/openshift-install
 
-FROM docker.io/opendevorg/python-base:3.8 as zuul
+FROM docker.io/opendevorg/python-base:3.7 as zuul
 ENV DEBIAN_FRONTEND=noninteractive
 
 COPY --from=builder /output/ /output