We need to move NixOS containers somewhere else so these don't clash
with Podman, Skopeo & other container software in the libpod &
cri-o/cri-u/libcontainer ecosystems.
The state directory move is not strictly a requirement but is good for
consistency.
the default hasn't been changed since 2009
this can improve our test performances
nixos/tests: remove explicit memorySize <1024
1024MiB is now the default
pathsInNixDB isn't a very accurate name when a Nix store image is
built (virtualisation.useNixStoreImage); rename it to additionalPaths,
which should be general enough to cover both cases.
Allows one or more directories to be mounted as a read-only file system.
This makes it convenient to run volatile containers that do not retain
application state.
