[docker] drop the bits argument when generating an ed25519 key (#6504)

From the man page of ssh-keygen:

  Ed25519 keys have a fixed length and the -b flag will be ignored.

[skip ci]

Signed-off-by: Jakob Ackermann <das7pad@outlook.com>

docker/etc/s6/openssh/setup

@@ -6,7 +6,7 @@ fi
 
 if [ ! -f /data/ssh/ssh_host_ed25519_key ]; then
     echo "Generating /data/ssh/ssh_host_ed25519_key..."
-    ssh-keygen -t ed25519 -b 4096 -f /data/ssh/ssh_host_ed25519_key -N "" > /dev/null
+    ssh-keygen -t ed25519 -f /data/ssh/ssh_host_ed25519_key -N "" > /dev/null
 fi
 
 if [ ! -f /data/ssh/ssh_host_rsa_key ]; then