Specifically use python 3.7 base images

We are consuming OpenDev's default python-builder and python-base images
which happen to be python3.7 today. Zuul specifically wants python3.7 so
we should explicitly use OpenDev's 3.7 tags of these images. This way if
OpenDev updates the default to 3.8 zuul can continue to assert its
dependency on 3.7.

Change-Id: I1f344eccfddff9bf1050c571604038371eec1a3f

.zuul.yaml

@@ -130,8 +130,8 @@
     allowed-projects: zuul/zuul
     timeout: 2700  # 45 minutes
     requires:
-      - python-builder-container-image
-      - python-base-container-image
+      - python-builder-3.7-container-image
+      - python-base-3.7-container-image
     provides: zuul-container-image
     vars: &zuul_image_vars
       docker_images:
@@ -160,8 +160,8 @@
     description: Build Docker images and upload to Docker Hub.
     allowed-projects: zuul/zuul
     requires:
-      - python-builder-container-image
-      - python-base-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 as builder
+FROM docker.io/opendevorg/python-builder:3.7 as builder
 
 # Optional location of Zuul API endpoint.
 ARG REACT_APP_ZUUL_API
@@ -39,7 +39,7 @@ 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 as zuul
+FROM docker.io/opendevorg/python-base:3.7 as zuul
 
 COPY --from=builder /output/ /output
 RUN /output/install-from-bindep \