Use a full entrypoint instead of a runAsRoot script for minecraft

This needs to be done because the server does not support qemu, and
NixOS requires qemu to use the runAsRoot feature for docker images.

Instead, create the required files as part of the entrypoint.

Fixes #6

etc/nixos/services/minecraft.nix

@@ -1,6 +1,14 @@
 { pkgs, ... }:
 
 let
+  entrypoint = pkgs.writeScript "entrypoint.sh" ''
+                           #!${pkgs.bash}/bin/bash
+                           ${pkgs.busybox}/bin/mkdir -p /var/lib/
+                           ${pkgs.gzip}/bin/gzip -dc ${./configs/minecraft.tar.gz} | ${pkgs.gnutar}/bin/tar -xf - -C /var/lib
+                           echo 'eula=true' > /var/lib/minecraft/eula.txt
+                           cp -f ${properties} /var/lib/minecraft
+                           $@
+                         '';
   ops = pkgs.writeText "ops.json" (builtins.toJSON [
     {
       uuid = "140d177a-966f-41b8-a4c0-e305babd291b";
@@ -72,13 +80,8 @@ in
       imageFile = pkgs.dockerTools.buildImage {
         name = "tlaternet/minecraft";
         tag = "latest";
-        runAsRoot = ''
-          mkdir -p /var/lib/
-          ${pkgs.gzip}/bin/gzip -dc ${./configs/minecraft.tar.gz} | ${pkgs.gnutar}/bin/tar -xf - -C /var/lib
-          echo 'eula=true' > /var/lib/minecraft/eula.txt
-          cp -f ${properties} /var/lib/minecraft
-        '';
         config = {
+          Entrypoint = [ "${entrypoint}" ];
           Cmd = [ "${pkgs.jre_headless}/bin/java"
                   "-Xms2G"
                   "-Xmx2G"