Give a sandbox a key without typing it into a command

In the CLI, --env NAME with no = sends that variable's value from your own shell, as docker run -e NAME does, on create, update, exec and run (withruntime 0.17.1). The security guide now shows how to install a key your sandbox's server checks on incoming requests, and the same key at another provider, and how to rotate it with no failed request. See Shared keys.