test: use quay mirror for the postgres container

The official postgres container image is fetched from docker.io,
which recently introduced rate limiting, which makes introduces
possible CI error if we run into this limit.
Instead use a custom mirror[1] of the official image, hosted on
quay.io[2]. As a side effect this updates the posgres version
from 12 to 13.

[1] osbuild/containers@7db3c68
[2] quay.io/osbuild/postgres:v1

test/run-koji-container.sh

@@ -69,7 +69,7 @@ koji_start() {
     -e POSTGRES_USER=koji \
     -e POSTGRES_PASSWORD=kojipass \
     -e POSTGRES_DB=koji \
-    docker.io/library/postgres:12-alpine
+    quay.io/osbuild/postgres:v1
 
   ${CONTAINER_RUNTIME} run -d --name org.osbuild.koji.kdc \
     --hostname org.osbuild.koji.kdc \