Use commands for container push

[bangelic]

Containers are able to be pushed to Satellite's container registry. This command for pushing containers was needed to show the user how to do so.

• [x] I am familiar with the contributing guidelines.

Please cherry-pick my commits into:

• [ ] Foreman 3.10/Katello 4.12
• [ ] Foreman 3.9/Katello 4.11 (Satellite 6.15; orcharhino 6.8)
• [ ] Foreman 3.8/Katello 4.10
• [ ] Foreman 3.7/Katello 4.9 (Satellite 6.14)
• [ ] Foreman 3.6/Katello 4.8
• [ ] Foreman 3.5/Katello 4.7 (Satellite 6.13; orcharhino 6.6/6.7)
• [ ] Foreman 3.4/Katello 4.6 (EL8 only)
• [ ] Foreman 3.3/Katello 4.5 on EL7 & EL8 (Satellite 6.12 on EL8 only; orcharhino 6.4/6.5 on EL8 only)
• We do not accept PRs for Foreman older than 3.3.