add /nix mount service

Dockerfile

@@ -48,6 +48,9 @@ COPY etc /etc
 # Copy /usr
 COPY usr /usr
 
+# Enable /nix mount service
+RUN systemctl enable mount-nix-prepare.service
+
 # Clear cache, /var and /tmp and commit ostree
 RUN rm -rf /tmp/* /var/* && mkdir -p /var/tmp && chmod -R 1777 /var/tmp && \
 ostree container commit

etc/systemd/system/mount-nix-prepare.service

@@ -0,0 +1,20 @@
+[Unit]
+Description=Prepare nix mount points
+
+[Service]
+Type=oneshot
+ExecStartPre=chattr -i /
+ExecStart=/bin/sh -c "mkdir -p /nix"
+ExecStart=/bin/sh -c "mkdir -p /var/lib/nix"
+ExecStart=/bin/sh -c "mount --bind /var/lib/nix /nix"
+# We need to restart systemd and load the nix-daemon again
+# as the nix-daemon files are actually inside the /nix folder
+# which hasn't been mounted until now
+ExecStart=/bin/sh -c "systemctl daemon-reload"
+# This will fail the first time as we haven't installed nix yet
+ExecStart=-/bin/sh -c "systemctl start nix-daemon.socket"
+ExecStopPost=chattr +i /
+
+[Install]
+WantedBy=local-fs.target
+