Pin docker images to 3.7 explicitly

We have versioned base images now. Pin to 3.7 (the current default)
so that we're explicit. We can update to 3.8 in a followup if we
want to.

Depends-On: https://review.opendev.org/714532
Change-Id: I6f92682b2d7c402af0a77183a71a6fdb2a1fac7d

.zuul.yaml

@@ -154,8 +154,8 @@
     dependencies:
       - opendev-buildset-registry
     requires:
-      - python-builder-container-image
-      - python-base-container-image
+      - python-builder-3.7-container-image
+      - python-base-3.7-container-image
     provides: nodepool-container-image
     vars: &nodepool_image_vars
       docker_images:
@@ -174,8 +174,8 @@
     parent: opendev-upload-docker-image
     description: Build Docker images and upload to Docker Hub.
     requires:
-      - python-builder-container-image
-      - python-base-container-image
+      - python-builder-3.7-container-image
+      - python-base-3.7-container-image
     provides: nodepool-container-image
     vars: *nodepool_image_vars
     secrets:
@@ -204,8 +204,8 @@
       - openstack/diskimage-builder
       - openstack/openstacksdk
     requires:
-      - python-builder-container-image
-      - python-base-container-image
+      - python-builder-3.7-container-image
+      - python-base-3.7-container-image
     provides: nodepool-siblings-container-image
     vars:
       docker_images:

Dockerfile

@@ -13,14 +13,14 @@
 # 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
 # ============================================================================
 
 ARG ZUUL_SIBLINGS=""
 COPY . /tmp/src
 RUN assemble
 
-FROM docker.io/opendevorg/python-base as nodepool-base
+FROM docker.io/opendevorg/python-base:3.7 as nodepool-base
 # ============================================================================
 
 COPY --from=builder /output/ /output