Make it possible to disable OpenSSH server in Docker image

Currently Omnibus GitLab requires that OpenSSH be managed with the Docker image even if gitlab-sshd is enabled. As explained in https://docs.gitlab.com/update/versions/gitlab_18_changes/#prng-is-not-seeded-error-on-docker-installations, OpenSSH may not start on a FIPS host because Ubuntu 24.04 ships with a version of OpenSSL fails when attempting to initiate FIPS on a non-FIPS image.

We should make it possible to disable OpenSSH if gitlab-sshd is used.

Relates to https://gitlab.com/gitlab-com/request-for-help/-/issues/2920

Acceptance Criteria

• Add a environment variable to disable OpenSSH on demand in Docker installs
• By default, OpenSSH remains enabled