Docker: Explain how to update with volumes and Compose (#9480)

Closes #4942

- Explain in *How to update* that data and extensions stay in the Docker volumes, so updating only means running a newer image with the same volumes, and recommend a back-up first.
- Add the Docker Compose variant (`pull` then `up` with the same `-f` files).
- Warn to keep the same major version of the PostgreSQL image when updating FreshRSS (a new major version refuses to start with older data), pointing to *Migrate database*, and to check `.env` variable names when replacing Compose files with newer copies.

Docker/README.md

@@ -105,6 +105,10 @@ and with newer packages in general (Apache, PHP).
 
 ## How to update
 
+Your data and extensions are kept in the `freshrss_data` and `freshrss_extensions` volumes,
+so updating FreshRSS only means running a newer image with the same volumes.
+Back up these volumes (and your database, if not using SQLite) first; see [Backup](../docs/en/admins/05_Backup.md).
+
 ```sh
 # Rebuild an image (see build section below) or get a new online version:
 docker pull freshrss/freshrss:edge
@@ -117,6 +121,17 @@ docker run ... --name freshrss freshrss/freshrss:edge
 docker rm freshrss_old
 ```
 
+With [Docker Compose](#docker-compose), run `pull` and then `up` again, with the same `-f` files as when you started, e.g.:
+
+```sh
+docker compose -f docker-compose.yml -f docker-compose-db.yml pull
+docker compose -f docker-compose.yml -f docker-compose-db.yml -f docker-compose-local.yml up -d --remove-orphans
+```
+
+> ⚠️ Keep the same major version of your database image (e.g. `postgres:17`) when updating FreshRSS, even if our example Compose files use a newer one:
+> a new major version of PostgreSQL refuses to start with the data of a previous one. To change it, see [Migrate database](#migrate-database).
+> If you replace your Compose files with newer copies of ours, also check that your `.env` file still uses the same variable names.
+
 ## Build custom Docker image
 
 Building your own Docker image is especially relevant for platforms not available on our Docker Hub,