volume: no need to duplicate the initial nix store

This is a partial revert of #7 where we added a copy of the initial
nix store to make sure it's not lost when we use a volume for the /nix
mounting point in the container.

However this was an unecessary duplication.

The initial nix store is already present in the docker image and can be
retrieved by using a “named” volume with `-v nix-store:/nix`, docker
will copy the /nix path of the image inside a volume called
`nix-store` on the host machine for us we don't need to it manually.

Of course if mounting an empty directory from the host machine (with
`-v /tmp/nix-store:/nix`) then the image won't boot (because docker
will NOT copy the content of the /nix path inside the image into the
/tmp/nix-store path of the host machine).

This change drastically reduces the image size (basically /2 as we
were duplicating the biggest source of disk space…)

```
REPOSITORY          TAG                 IMAGE ID            CREATED              SIZE
docker-nix          2.3.4-vol           f74209e25515        About a minute ago   1.58GB
docker-nix          2.3.4               61880484ceb1        12 minutes ago       835MB
```

Let's be 🍃 instead of ⛰️

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