Add instructions for installing on Rancher in air-gapped environment

[LukasK32]

Description

Adds instructions for installing on Rancher in air-gapped environment.

How Has This Been Tested?

Manually installed using new instructions.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[keyaray]

LGTM!