fix(BREV-2599): Ensure nebius makes use of iptables rules in addition to ufw

[drewmalin]

No description provided.

[Copilot]

Pull request overview

This PR ensures that Nebius provider instances have proper Docker container security by implementing iptables rules alongside UFW firewall rules, preventing Docker containers from being immediately accessible from the internet by default.

Changes:

• Added iptables rules to the Nebius provider to secure Docker containers by default
• Removed an extraneous comment line from the Shadeform provider's firewall configuration

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
v1/providers/shadeform/firewall.go	Removed empty comment line for code cleanliness
v1/providers/nebius/instance.go	Added generateIPTablesCommands function and integrated iptables rules into cloud-init user data generation

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[theFong]

Looks good! For more confidence we can merge in:
#81

Then see the failing test pass.