The instance cannot be reused by another user

[Egor-S]

Since SSH keys are added to the instance only during creation, other users cannot add their keys later. dstack server must update SSH keys on job submission (docker image submission, to be precise). There are multiple ways to do so:

1. Write directly to the /home/ubuntu/.ssh/authorized_keys
2. Delegate this task to the shim (or to the runner in container computes)

Additionally, we should consider removing old keys from the instance to match the behavior with brand-new instances.