3305 - Make sure pender works with PostgreSQL 12 (#360)

* update postgres to 12

we are having an issue with building on Travis that seems related
to a change the maintainers of the postgres docker images have made to
the underlying OS image layer:

Previous: Debian 11 (bullseye) New: Debian 12 (bookworm)

The workaround seems to be using postgres-bullseye

more on this here:
https://stackoverflow.com/a/76591040

docker-compose.yml

@@ -2,7 +2,7 @@ version: "2.2"
 volumes:
   redis:
   minio:
-  postgres11:
+  postgres12:
 services:
   redis:
     image: redis:5
@@ -21,7 +21,7 @@ services:
       MINIO_ACCESS_KEY: AKIAIOSFODNN7EXAMPLE
       MINIO_SECRET_KEY: wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY
   postgres:
-    image: postgres:11
+    image: postgres:12-bullseye
     ports:
       - "5432:5432"
     environment: