Move .dockerenv parsing to lxc driver

This is not needed for e.g. the native driver

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Move all bind-mounts in the container inside the namespace

This moves the bind mounts like /.dockerinit, /etc/hostname, volumes,
etc into the container namespace, by setting them up using lxc.

This is useful to avoid littering the global namespace with a lot of
mounts that are internal to each container and are not generally
needed on the outside. In particular, it seems that having a lot of
mounts is problematic wrt scaling to a lot of containers on systems
where the root filesystem is mounted --rshared.

Note that the "private" option is only supported by the native driver, as
lxc doesn't support setting this. This is not a huge problem, but it does
mean that some mounts are unnecessarily shared inside the container if you're
using the lxc driver.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Don't use separate bind mount for container

Since we're not not mounting anything but the base filesystem outside
the container we no longer need the separate bind mount at
/var/lib/docker/container/$id/root in order to see the base filesystem
without extra mounts. So, we drop this and mount (again) the container
root directly at the real basefs mountpoint.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #4422 from alexlarsson/internal-mounts

Move all bind-mounts in the container inside the namespace