Document the Alpine 3.24 runtime image (#48)

docs/backup-export.md

@@ -29,8 +29,10 @@ changes, PostgreSQL roles and cluster settings, Redis and deployment files are n
 - `pg_dump` must be in the **same runtime** as the exporter (the CLI's environment, or the
   server's PATH for the download), with a major version at least as new as the database's. A client on
   the Docker host or in another container doesn't count.
-- The Docker image ships the PostgreSQL 16 client. Its Alpine 3.19 base has no 17/18
-  packages, so a newer database needs a different runtime, not just `POSTGRES_CLIENT_MAJOR`.
+- The published Docker image ships the PostgreSQL 16 client on an Alpine 3.24 base. For a
+  PostgreSQL 17 or 18 database, build your own image from a beacon-server checkout, e.g.
+  `docker build -f .build/Dockerfile --build-arg POSTGRES_CLIENT_MAJOR=17 -t beacon:pg17 .`,
+  and point the stack's `image:` at that tag.
 - The temp directory (`TMPDIR`) needs about twice the SQL limit free. A small tmpfs won't do.
 - The output filesystem must support hard links (used to publish atomically). Existing
   destinations, symlinks included, are never replaced. Files are 0600 and staging dirs 0700;

docs/upgrading.md

@@ -333,6 +333,13 @@ instead of an `ERROR` pair; a real `500` still logs `api: error 500: <message>`
 with the database error in `error`. Only pages that read a read model count timeouts in
 `beacon_api_unavailable_total{reason="timeout"}`.
 
+#### Runtime image on Alpine 3.24
+
+The server image moved from Alpine 3.19, which is out of support, to Alpine 3.24 with the
+PostgreSQL 16 client tools. Nothing to change for a PostgreSQL 16 database. A self-built image
+with `POSTGRES_CLIENT_MAJOR=17` or `18` now works too; see
+[Backup export](backup-export.md).
+
 ## Changes that need a restart
 
 Beacon reads `config.yaml` once at startup. After editing it, run