fix(test): generate RSA SSH key in docker-popular build

The SSH connection options require RSA keys
(PubkeyAcceptedAlgorithms=rsa-sha2-512), but build_rootfs.sh
was using the default key type (ED25519 on modern OpenSSH).
This mismatch caused SSH authentication failures in the
docker-popular pipeline.

Align with setup-ci-artifacts.sh by explicitly generating
RSA-2048 keys.

Signed-off-by: Riccardo Mancini <mancio@amazon.com>

Author: Manciukic

tools/test-popular-containers/build_rootfs.sh

@@ -22,7 +22,7 @@ IMAGES=(amazonlinux:2023 alpine:latest ubuntu:22.04 ubuntu:24.04 ubuntu:25.04 ub
 
 # Generate SSH key for access from host
 if [ ! -s id_rsa ]; then
-  ssh-keygen -f id_rsa -N ""
+  ssh-keygen -t rsa -b 2048 -f id_rsa -N ""
 fi
 
 # install rootfs dependencies