temmie: combine homedirs in overlayfs

hosts/temmie/services/nfs-mounts.nix

@@ -8,7 +8,8 @@ in
     description = "PVV Homedir Partitions";
   };
 
-  systemd.mounts = map (l: {
+  systemd.mounts =
+    (map (l: {
       description = "PVV Homedir Partition ${l}";
 
       before = [ "remote-fs.target" ];
@@ -56,5 +57,20 @@ in
         # TODO: can we enable this and still run cgi stuff?
         # "noexec"
       ];
-  }) letters;
+    }) letters)
+    ++ [{
+      description = "PVV Merged Homedir OverlayFS";
+
+      after = [ "remote-fs.target" ];
+      wantedBy = [ "multi-user.target" ];
+      requiredBy = [ "pvv-homedirs.target" ];
+
+      type = "overlay";
+      what = "overlay";
+      where = "/run/pvv-home-mounts-merged";
+
+      options = lib.concatStringsSep "," [
+        "lowerdir=${lib.concatMapStringsSep ":" (l: "/run/pvv-home-mounts/${l}") letters}"
+      ];
+    }];
 }