Be explicit about python version in image

python-base has versions available now, defaulting to 3.7. Update
our config to 3.7 to be explicit about what we're using. This will
let us update the version as we feel like.

Change-Id: I40ffde91808a8bb95479697b9127dba16de8a8cd

.zuul.yaml

@@ -174,8 +174,8 @@
     description: Build Docker images.
     allowed-projects: openstack/python-openstackclient
     requires:
-      - python-builder-container-image
-      - python-base-container-image
+      - python-builder-3.7-container-image
+      - python-base-3.7-container-image
     provides: osc-container-image
     vars: &osc_image_vars
       docker_images:
@@ -188,8 +188,8 @@
     description: Build Docker images and upload to Docker Hub.
     allowed-projects: openstack/python-openstackclient
     requires:
-      - python-builder-container-image
-      - python-base-container-image
+      - python-builder-3.7-container-image
+      - python-base-3.7-container-image
     provides: osc-container-image
     secrets:
       - name: docker_credentials

Dockerfile

@@ -13,12 +13,12 @@
 # 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
 
 COPY . /tmp/src
 RUN assemble
 
-FROM docker.io/opendevorg/python-base
+FROM docker.io/opendevorg/python-base:3.7
 
 COPY --from=builder /output/ /output
 RUN /output/install-from-bindep