Under the hood the creation of an environment corresponds to the creation of a docker container (that's how it relates to docker). The docker container is built starting from a the official python docker images as you can see here https://github.com/se7entyse7en/pydockenv/blob/master/pydock....
The activation of the environment corresponds to the start of the container. Hence inside the container, you don't have (don't need) to have any virtual env as the isolation is already provided by the container itself.
Every time you install a dependency, this is actually installed inside the container.