Docker execution mode "session" prevents variable/file injection via authz/config

## What would you like to add/change in the documentation?

This is directed at git tag v0.5.

Neither [docs/reference/image.md](https://github.com/ContainerSSH/containerssh.github.io/blob/v0.5/docs/reference/image.md), nor the "Other options" section of [docs/reference/docker.md](https://github.com/ContainerSSH/containerssh.github.io/blob/v0.5/docs/reference/docker.md) mention that setting the `docker:execution:mode` to `session` will forgo injection of environment variables or files.

This took me some time to chase down, as I assumed it was my configuration server response, then I assumed it was how I built my guest image, then I tried switching from a configuration server to an authorization server. There is no warning, info, or debug statement in the logs that authz/config options returned in the response are ignored or incompatible with other configuration settings, which I suppose might be a feature request, separate from this issue.

## Where did you try to find the information?

I would expect a warning/caveat/note to be presented in either:
* [docs/reference/image.md](https://github.com/ContainerSSH/containerssh.github.io/blob/v0.5/docs/reference/image.md)
* [docs/reference/docker.md](https://github.com/ContainerSSH/containerssh.github.io/blob/v0.5/docs/reference/docker.md)