Docker Environment

[blaubaer]

Is your feature request related to a problem? Please describe.

1. I do not want to expose the hosts file-system to all of my users. Especially if this host is used as a Bastion Host / Jump Host.
2. I want easy and different propagation of different root file systems for different users.
3. I don't want to install all required tool on the hosts file system.

Describe the solution you'd like

Instead of the currently supported local environment, we should also add a docker environment which will run every Bifröst's session inside an own docker container.

It should be possible to use different:

1. Environment variables
2. Images
3. Networks
4. Commands
5. Container internal users