Update to nix version 2.2.1.

The new version has sandboxed builds enabled by default, which
won't work in an unprivileged docker container, so we disable
that feature by way of /etc/nix/nix.conf.

Closes #6.

Dockerfile

@@ -6,11 +6,13 @@ FROM alpine
 RUN apk add --no-cache --update openssl
 
 # Download Nix and install it into the system.
-RUN wget https://nixos.org/releases/nix/nix-2.1.3/nix-2.1.3-x86_64-linux.tar.bz2 \
-  && echo "3169d05aa713f6ffa774f001cae133557d3ad72e23d9b6f6ebbddd77b477304f  nix-2.1.3-x86_64-linux.tar.bz2" | sha256sum -c \
+RUN wget https://nixos.org/releases/nix/nix-2.2.1/nix-2.2.1-x86_64-linux.tar.bz2 \
+  && echo "e229e28f250cad684c278c9007b07a24eb4ead239280c237ed2245871eca79e0  nix-2.2.1-x86_64-linux.tar.bz2" | sha256sum -c \
   && tar xjf nix-*-x86_64-linux.tar.bz2 \
   && addgroup -g 30000 -S nixbld \
   && for i in $(seq 1 30); do adduser -S -D -h /var/empty -g "Nix build user $i" -u $((30000 + i)) -G nixbld nixbld$i ; done \
+  && mkdir -m 0755 /etc/nix \
+  && echo 'sandbox = false' > /etc/nix/nix.conf \
   && mkdir -m 0755 /nix && USER=root sh nix-*-x86_64-linux/install \
   && ln -s /nix/var/nix/profiles/default/etc/profile.d/nix.sh /etc/profile.d/ \
   && rm -r /nix-*-x86_64-linux* \