chore(contrib): pin postgres to 18 in Docker Compose examples

postgres:latest is expected to move to PostgreSQL 19 on its planned
release date, November 19, 2026. The 19 release candidate image exits
before starting on the PostgreSQL 18 data these files keep at
/var/lib/postgresql. postgres:18 is the same image as latest today and
matches the Docker guide on the website.

.devcontainer/docker-compose.yml

@@ -10,7 +10,7 @@ services:
       - ADMIN_USERNAME=admin
       - ADMIN_PASSWORD=test123
   db:
-    image: postgres:latest
+    image: postgres:18
     restart: unless-stopped
     volumes:
       - postgres-data:/var/lib/postgresql

contrib/docker-compose/basic.yml

@@ -19,7 +19,7 @@ services:
     # healthcheck:
     #  test: ["CMD", "/usr/bin/miniflux", "-healthcheck", "auto"]
   db:
-    image: postgres:latest
+    image: postgres:18
     container_name: postgres
     restart: always
     environment:

contrib/docker-compose/caddy.yml

@@ -25,7 +25,7 @@ services:
       - ADMIN_PASSWORD=test123
       - BASE_URL=https://miniflux.example.org
   db:
-    image: postgres:latest
+    image: postgres:18
     container_name: postgres
     environment:
       - POSTGRES_USER=miniflux

contrib/docker-compose/traefik.yml

@@ -37,7 +37,7 @@ services:
       - "traefik.http.routers.miniflux.entrypoints=websecure"
       - "traefik.http.routers.miniflux.tls.certresolver=myresolver"
   db:
-    image: postgres:latest
+    image: postgres:18
     container_name: postgres
     environment:
       - POSTGRES_USER=miniflux