[DOCS] Document SSH key mounts into Docker volumes

[eliashaeussler]

Is your feature request related to a problem?

When creating projects using the provided Docker image, it may be necessary to mount SSH keys into the container. Otherwise, private template packages may not be accessible.

Mounting SSH keys is a potentially complex task, because it requires a deeper knowledge of Docker and its mounting strategies.

Describe the solution you'd like

We should add a chapter to the Docker usage section in the documentation. It should describe how to mount SSH keys into Docker containers.

Describe alternatives you've considered

It might also be necessary to improve handling of SSH keys in Docker container code-wise a little bit.

Additional context

No response