Add nginx-ssh image: nginx + sshd for external SSH ingress testing

[mmnabeel317]

Summary

Adds a new nginx-ssh image — a variant of the existing nginx image with OpenSSH server added. Used by kube-burner-ocp's cudn-density workload to validate external SSH ingress to BGP-advertised CUDN pod IPs.

Closes cloud-bulldozer/kube-burner-ocp#491

Changes

• nginx-ssh/Dockerfile: Based on ubi9/ubi-minimal, installs nginx, openssh-server, openssh-clients, shadow-utils; creates a non-root sshtest user and /etc/ssh-authorized-keys directory for runtime key injection
• nginx-ssh/root/etc/ssh/sshd_config: Configures sshtest for pubkey-only auth via a mounted AuthorizedKeysFile; sets StrictModes no globally (required because Kubernetes Secret volumes mount directories as 1777)
• nginx-ssh/root/entrypoint.sh: Starts sshd then nginx (foreground)
• nginx-ssh/root/etc/nginx/: Standard nginx config with TLS on 8443 and HTTP on 8080
• nginx-ssh/root/usr/share/nginx/html/128.html: Minimal response payload for nginx health checks
• Makefile: Added nginx-ssh to the build targets

Design decisions

• Kept the full nginx stack (TLS/8443, mime.types, etc.) rather than a minimal nginxecho-style image, since the cudn-density workload's existing client/app pods make HTTP/HTTPS requests to the server — SSH is additive, not a replacement
• No SSH credentials baked into the image; the authorized key is mounted at runtime via a Kubernetes Secret
• Only 128.html is included (all other size-variant HTML files removed as unused by the workload)

Test plan

• Verified image builds and runs locally with podman
• Verified SSH login works as sshtest with a mounted public key (on baremetal OpenShift cluster)
• Verified end-to-end with cudn-density --bgp (20/20 SSH checks passed)

/cc @venkataanil