Provide public host key or known hosts when using the SSH backend

Up to version 2.49.1 the image would not check the SSH host’s identity, which is unsafe. This potentially leaks credentials or backup content to external attackers.

Starting with version 2.49.2, this is prevented, requiring you to either:

• provide the host’s public key by setting either SSH_HOST_PUBLIC_KEY (the key’s content) or SSH_HOST_PUBLIC_KEY_FILE (the fs location of the key in a file)
• provide the location of a valid known_hosts file by setting SSH_KNOWN_HOSTS_FILE
• dangerously set SSH_INSECURE_IGNORE_HOST_KEY to ignore host key checks

Only set SSH_INSECURE_IGNORE_HOST_KEY as a last resort or in entirely airgapped environments.