Docker: Make the reference to postgres image more specific

Podman does not have "postgres" in its default list of aliases.

Author: gollux

docker-compose.dev.yml

@@ -1,6 +1,6 @@
 services:
   devdb:
-    image: postgres:15
+    image: docker.io/postgres:15
     environment:
       POSTGRES_HOST_AUTH_METHOD: trust
     volumes:

docker-compose.test.yml

@@ -1,6 +1,6 @@
 services:
   testdb:
-    image: postgres:15
+    image: docker.io/postgres:15
     environment:
       POSTGRES_HOST_AUTH_METHOD: trust