install nix under the unprivileged user

etc/docker/ubuntu-with-gnupg/Dockerfile

@@ -1,42 +1,36 @@
 FROM fpco/ubuntu-with-libgmp:14.04
 
+# INSTALL PACKAGES
 ENV DEBIAN_FRONTEND noninteractive
 RUN apt-get update
 RUN apt-get -y install curl expect
 
-# SETUP A NIX BUILD GROUP & USER
+# CREATE USER
+ENV USER user
+ENV HOME /home/$USER
 RUN groupadd -r nixbld
-RUN useradd -c "Nix Build" -d /var/empty -g nixbld -G nixbld \
-    -M -N -r -s "$(which nologin)" nixbld
+RUN useradd -m $USER -G nixbld
+ADD .gnupg/ $HOME/.gnupg/
+RUN chown -R user:user $HOME
+RUN chmod -R go-rwsx $HOME
+RUN mkdir /nix
+RUN chown $USER:nixbld /nix
+WORKDIR $HOME
+USER $USER
 
 # INSTALL NIX
-RUN curl -s https://nixos.org/nix/install | USER=root sh
-RUN ln -s /nix/var/nix/profiles/default/etc/profile.d/nix.sh /etc/profile.d/
-RUN bash -lc "nix-channel --update"
+RUN curl -s https://nixos.org/nix/install | sh
+RUN echo "\nsource /nix/var/nix/profiles/default/etc/profile.d/nix.sh" \
+  | tee -a $HOME/.bash_profile
 
-# INSTALL ALL 3 VERSIONS OF GNUPG
+# INSTALL GNUPGS & SETUP KEYS
 RUN bash -lc "nix-env -p /nix/var/nix/profiles/gpg14 -iA nixpkgs.nix nixpkgs.gnupg1orig"
 RUN bash -lc "nix-env -p /nix/var/nix/profiles/gpg20 -iA nixpkgs.nix nixpkgs.gnupg20"
 RUN bash -lc "nix-env -p /nix/var/nix/profiles/gpg21 -iA nixpkgs.nix nixpkgs.gnupg"
-
-# MAKE GNUPG 1 THE DEFAULT
-RUN bash -lc "nix-env -S /nix/var/nix/profiles/gpg14"
-
-# CREATE USER w/ HOME DIR
-RUN useradd -m user
-ENV HOME /home/user
-ADD .gnupg/ $HOME/.gnupg/
-RUN chown -R user:user $HOME
-RUN chmod -R go-rwsx $HOME
-WORKDIR $HOME
-
-# SETUP GPG
-USER user
+RUN bash -lc "nix-env -S /nix/var/nix/profiles/gpg14" ;# SET CURRENT PROFILE TO GPG 1.4
 RUN bash -lc "gpg --import $HOME/.gnupg/public.asc $HOME/.gnupg/subkey.asc"
 RUN bash -lc "expect $HOME/.gnupg/trust"
 RUN rm $HOME/.gnupg/*.asc $HOME/.gnupg/trust
 
-# CLEANUP
-RUN apt-mark auto curl expect
-RUN apt-get -y autoremove
-RUN apt-get -y clean
+# LOGIN SHELL
+CMD /bin/bash -l