[buildbot] docker: bump rsync Debian version

[LEDE Commits]

ynezz pushed a commit to buildbot.git, branch main:
https://git.openwrt.org/f1a1a6264422bdfe42e1aae1ad33d866a5b51955

commit f1a1a6264422bdfe42e1aae1ad33d866a5b51955
Author: Paul Spooren <mail at aparcar.org>
AuthorDate: Wed Aug 7 16:49:50 2024 +0200

    docker: bump rsync Debian version
    
    Signed-off-by: Paul Spooren <mail at aparcar.org>
---
 docker/rsync/Dockerfile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/docker/rsync/Dockerfile b/docker/rsync/Dockerfile
index 4c8ca34..c2cf8ae 100644
--- a/docker/rsync/Dockerfile
+++ b/docker/rsync/Dockerfile
@@ -1,4 +1,4 @@
-FROM debian:9
+FROM debian:12
 
 COPY docker/rsync/files/entry.sh /entry.sh