chore(examples): add postgres 15 image tag

Signed-off-by: Josh <josh.t.richards@gmail.com>

.examples/docker-compose/insecure/postgres/apache/compose.yaml

 services:
   db:
-    image: postgres:alpine
+    image: postgres:15
     restart: always
     volumes:
       - db:/var/lib/postgresql/data:Z

.examples/docker-compose/insecure/postgres/fpm/compose.yaml

 services:
   db:
-    image: postgres:alpine
+    image: postgres:15
     restart: always
     volumes:
       - db:/var/lib/postgresql/data:Z

.examples/docker-compose/with-nginx-proxy/postgres/apache/compose.yaml

 services:
   db:
-    image: postgres:alpine
+    image: postgres:15
     restart: always
     volumes:
       - db:/var/lib/postgresql/data:Z

.examples/docker-compose/with-nginx-proxy/postgres/fpm/compose.yaml

@@ -2,7 +2,7 @@ version: '3'
 
 services:
   db:
-    image: postgres:alpine
+    image: postgres:15
     restart: always
     volumes:
       - db:/var/lib/postgresql/data:Z

README.md

@@ -395,7 +395,7 @@ As an alternative to passing sensitive information via environment variables, `_
 ```yaml
 services:
   db:
-    image: postgres
+    image: postgres:15
     restart: always
     volumes:
       - db:/var/lib/postgresql/data