Update ptgbot's docker image to bullseye

Bullseye has been out for a bit now and buster has a shelf life. Update
to bullseye to keep up with debian's releases.

Change-Id: I5845a8cf5c7a0c6383e9c2863146df28c7e40a4f

.zuul.yaml

@@ -18,6 +18,9 @@
     name: ptgbot-build-opendev-image
     parent: opendev-build-docker-image
     description: Build OpenDev Docker images for statusbot
+    requires:
+      - python-builder-3.9-bullseye-container-image
+      - python-base-3.9-bullseye-container-image
     provides: ptgbot-container-image
     vars: &ptgbot_opendev_image_vars
       zuul_work_dir: src/opendev.org/openstack/ptgbot
@@ -30,6 +33,9 @@
     name: ptgbot-upload-opendev-image
     parent: opendev-upload-docker-image
     description: Build OpenDev statusbot Docker images and upload to Docker Hub.
+    requires:
+      - python-builder-3.9-bullseye-container-image
+      - python-base-3.9-bullseye-container-image
     provides: ptgbot-container-image
     vars: *ptgbot_opendev_image_vars
     secrets:

Dockerfile

@@ -1,10 +1,10 @@
 
-FROM opendevorg/python-builder:3.9 as builder
+FROM opendevorg/python-builder:3.9-bullseye as builder
 
 COPY . /tmp/src
 RUN assemble
 
-FROM opendevorg/python-base:3.9 as ptgbot
+FROM opendevorg/python-base:3.9-bullseye as ptgbot
 
 COPY --from=builder /output/ /output
 RUN /output/install-from-bindep