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 ⛰️

3 files changed, 0 insertions, 9 deletions

diff --git a/alpine/Dockerfile b/alpine/Dockerfile
index 0e42345..1da307a 100644
--- a/alpine/Dockerfile
+++ b/alpine/Dockerfile
@@ -40,9 +40,6 @@ RUN nix-env -iA nixpkgs.glibcLocales
 
 # < Nix context as a volume
 # We want to be able to define /nix/store as a volume
-# We thus need to "save" the current nix context to be able
-# to restore it at startup time
-RUN cp -R /nix /home/nixuser/initial-nix
 VOLUME ["/nix"]
 # Create bash profile
 COPY --chown=nixuser:nixuser files/.profile ${HOME}/.profile
diff --git a/debian/Dockerfile b/debian/Dockerfile
index 90b2e65..7e4eaea 100644
--- a/debian/Dockerfile
+++ b/debian/Dockerfile
@@ -40,9 +40,6 @@ RUN nix-env -iA nixpkgs.glibcLocales
 
 # < Nix context as a volume
 # We want to be able to define /nix/store as a volume
-# We thus need to "save" the current nix context to be able
-# to restore it at startup time
-RUN cp -R /nix /home/nixuser/initial-nix
 VOLUME ["/nix"]
 # Create bash profile
 COPY --chown=nixuser:nixuser files/.profile ${HOME}/.profile
diff --git a/files/.profile b/files/.profile
index 68b4473..2fadec2 100755
--- a/files/.profile
+++ b/files/.profile
@@ -1,8 +1,5 @@
 #!/usr/bin/env bash
 
-# Copy initial saved nix context if the /nix is empty
-[ ! "$(ls -A /nix)" ] && cp -R ~/initial-nix/* /nix/
-
 # Source nix environment
 nix_profile="/home/nixuser/.nix-profile/etc/profile.d/nix.sh"
 # shellcheck source=/home/nixuser/.nix-profile/etc/profile.d/nix.sh