Update the docker image to python3.9 and buster

This is long overdue now that buster is out and python3.9 has been
around for a bit. This update allows us to remove the buster 3.7 images
that the gear images were based on.

Depends-On: https://review.opendev.org/c/opendev/gear/+/838562
Change-Id: I8d57c06fe28276d6e15934a785b1f97125f0958f

.zuul.yaml

@@ -19,8 +19,8 @@
     parent: opendev-build-docker-image
     description: Build OpenDev Docker images for gear
     requires:
-      - python-base-container-image
-      - python-builder-container-image
+      - python-base-3.9-bullseye-container-image
+      - python-builder-3.9-bullseye-container-image
     provides: gear-container-image
     vars: &gear_opendev_image_vars
       zuul_work_dir: src/opendev.org/opendev/gear
@@ -34,8 +34,8 @@
     parent: opendev-upload-docker-image
     description: Build OpenDev gear Docker images and upload to Docker Hub.
     requires:
-      - python-base-container-image
-      - python-builder-container-image
+      - python-base-3.9-bullseye-container-image
+      - python-builder-3.9-bullseye-container-image
     provides: gear-container-image
     vars: *gear_opendev_image_vars
     secrets:

Dockerfile

@@ -13,12 +13,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM opendevorg/python-builder as builder
+FROM opendevorg/python-builder:3.9-bullseye as builder
 
 COPY . /tmp/src
 RUN assemble
 
-FROM opendevorg/python-base as geard
+FROM opendevorg/python-base:3.9-bullseye as geard
 
 COPY --from=builder /output/ /output
 RUN /output/install-from-bindep