Add the INTERFACE env var to allow specifying the interface to use in the container

[tanty]

This enables using the container with rootless podman.

rootless podman uses slirp4netns which provides the "tap0" interface in the container, while not having acces to the "eth0" interface.

You can test this, running with rootless podman, like:

$ podman run -d \
  --cap-add=NET_ADMIN #required \
  --cap-add=NET_RAW #required \
  -e PRIVATE_KEY=xxxxxxxxx #required \
  -e INTERFACE=tap0  \
  docker.io/tanty/nordlynx:rootless-podman

[bubuntux]

thanks for the contribution