use postgres 17 for new installations, add upgrade hint (#147)

.env.example

@@ -24,3 +24,4 @@ PGDATA="/var/lib/postgresql/data"
 OPDATA="/var/openproject/assets"
 COLLABORATIVE_SERVER_URL=ws://localhost:8080/hocuspocus
 COLLABORATIVE_SERVER_SECRET=secret12345
+POSTGRES_VERSION=17 # Note for existing setups: make sure this is the same version you are using right now, or upgrade your database

docker-compose.yml

@@ -33,7 +33,13 @@ x-op-app: &app
 
 services:
   db:
-    image: postgres:13
+    # The default here should really be 17 but is still 13 as to not break existing setups.
+    # New installations using the provided .env.example will use 17 to begin with.
+    # The default here will change to 17 as well with the next major update (18).
+    #
+    # Refer to the docs on how to upgrade to postgres 17 before then:
+    #   https://www.openproject.org/docs/installation-and-operations/misc/migration-to-postgresql17/
+    image: postgres:${POSTGRES_VERSION:-13}
     <<: *restart_policy
     stop_grace_period: "3s"
     volumes: