Upgrade failure recovery documentation is incomplete

**What happened**:
The documentation on this page is unclear or incomplete. https://anywhere.eks.amazonaws.com/docs/tasks/troubleshoot/troubleshooting/#cluster-upgrade-fails-with-management-cluster-on-bootstrap-cluster

The `${CONTAINER}` environment variable is not defined and it is unclear what it should be set to.

**What you expected to happen**:
I expected all of the environment variables to come with clear instructions on how to set them.

**How to reproduce it (as minimally and precisely as possible)**:
Try and follow the doc :)

**Anything else we need to know?**:
❤️

Start with the linked troubleshooting page and the section for cluster upgrades failing with the management cluster on the bootstrap cluster. Review every environment variable there, especially `${CONTAINER}`, and update the documentation so each variable has clear instructions for its value and setup. Done means a newcomer can follow the recovery procedure without guessing any environment variable.

Written by the indexing model from the issue text.