chore(deps): update docker.io/library/postgres docker tag to v18

resources/chaosknoten/pad/docker_compose/compose.yaml.j2

@@ -3,7 +3,7 @@
 
 services:
   database:
-    image: docker.io/library/postgres:15-alpine
+    image: docker.io/library/postgres:18-alpine@sha256:f898ac406e1a9e05115cc2efcb3c3abb3a92a4c0263f3b6f6aaae354cbb1953a
     environment:
       - "POSTGRES_USER=hedgedoc"
       - "POSTGRES_PASSWORD={{ secret__hedgedoc_db_password }}"

resources/chaosknoten/pretalx/docker_compose/compose.yaml.j2

@@ -3,7 +3,7 @@
 
 services:
   database:
-    image: docker.io/library/postgres:15-alpine
+    image: docker.io/library/postgres:18-alpine@sha256:f898ac406e1a9e05115cc2efcb3c3abb3a92a4c0263f3b6f6aaae354cbb1953a
     environment:
       - "POSTGRES_USER=pretalx"
       - "POSTGRES_PASSWORD={{ secret__pretalx_db_password }}"

resources/chaosknoten/tickets/docker_compose/compose.yaml.j2

@@ -1,7 +1,7 @@
 ---
 services:
   database:
-    image: docker.io/library/postgres:15-alpine
+    image: docker.io/library/postgres:18-alpine@sha256:f898ac406e1a9e05115cc2efcb3c3abb3a92a4c0263f3b6f6aaae354cbb1953a
     environment:
       - "POSTGRES_USER=pretix"
       - "POSTGRES_PASSWORD={{ secret__pretix_db_password }}"