Instead reserve exit code 2 to be future proof, document that it should
not be used. Implementation-wise, it is considered as unhealthy, but
users should not rely on this as it may change in the future.
Signed-off-by: Tibor Vass <tibor@docker.com>
Upstream-commit: 91e9f3831330c63f8351b9fc3f7c31b3229505be
Component: engine