feat: add commit timestamp to oci-image

nix/pkgs/oci-image/default.nix

@@ -1,6 +1,7 @@
 # Keep sorted
 { default
 , dockerTools
+, inputs
 , lib
 , pkgs
 }:
@@ -13,10 +14,27 @@ let
       })
     ];
   };
+
+  # This is a unix timestamp of the last commit
+  timestamp = inputs.self.lastModified;
+
+  # Convert the timestamp to an ISO 8601 string
+  iso = builtins.readFile (
+    pkgs.runCommand "timestamp" {
+      when = timestamp;
+      nativeBuildInputs = [ pkgs.coreutils ];
+    } ''
+      date -u -d "@$when" +%Y-%m-%dT%H:%M:%SZ > $out
+    ''
+  );
+
+  # Remove trailing newline
+  isoTrimmed = lib.strings.trim iso;
 in
 dockerTools.buildImage {
   name = default.pname;
   tag = "next";
+  created = isoTrimmed;
   copyToRoot = [
     dockerTools.caCertificates
   ];