Build nodepool with python3.8

We updated the zuul images to py3.8, let's update the nodepool ones
too.

Change-Id: Id4b9c00d14c5fb0dd192760084c6563a8bae3636

.zuul.yaml

@@ -154,8 +154,8 @@
     dependencies:
       - opendev-buildset-registry
     requires:
-      - python-builder-3.7-container-image
-      - python-base-3.7-container-image
+      - python-builder-3.8-container-image
+      - python-base-3.8-container-image
     provides: nodepool-container-image
     vars: &nodepool_image_vars
       docker_images:
@@ -183,8 +183,8 @@
     parent: opendev-upload-docker-image
     description: Build Docker images and upload to Docker Hub.
     requires:
-      - python-builder-3.7-container-image
-      - python-base-3.7-container-image
+      - python-builder-3.8-container-image
+      - python-base-3.8-container-image
     provides: nodepool-container-image
     vars: *nodepool_image_vars
     secrets:
@@ -213,8 +213,8 @@
       - openstack/diskimage-builder
       - openstack/openstacksdk
     requires:
-      - python-builder-3.7-container-image
-      - python-base-3.7-container-image
+      - python-builder-3.8-container-image
+      - python-base-3.8-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:3.7 as builder
+FROM docker.io/opendevorg/python-builder:3.8 as builder
 # ============================================================================
 
 ARG ZUUL_SIBLINGS=""
 COPY . /tmp/src
 RUN assemble
 
-FROM docker.io/opendevorg/python-base:3.7 as nodepool-base
+FROM docker.io/opendevorg/python-base:3.8 as nodepool-base
 # ============================================================================
 
 COPY --from=builder /output/ /output