assume remote docker when running inside docker

When running inside a Docker container (which can be identified by the presence of /.dockerenv), it is not recommended to run dockerd inside the same Docker container ("dind"). It is more common to bind-mount /var/run/docker.sock into the container, allowing it to create new containers in parallel with the current one.

This is e.g. the case when we're running in a container on GitHub Actions.

In such setups, the container running autopkgtest is unlikely to have a shared filesystem with the container that autopkgtest spawns via virt-docker. Therefore we should avoid optimizations that rely on a shared filesystem.