factor out nix code into new files via makeScope

This makes the Nix code a lot easier to reason about.

nix/pkgs/oci-image/default.nix

@@ -0,0 +1,25 @@
+# Keep sorted
+{ default
+, dockerTools
+, lib
+, tini
+}:
+
+dockerTools.buildImage {
+  name = default.pname;
+  tag = "next";
+  copyToRoot = [
+    dockerTools.caCertificates
+  ];
+  config = {
+    # Use the `tini` init system so that signals (e.g. ctrl+c/SIGINT)
+    # are handled as expected
+    Entrypoint = [
+      "${lib.getExe' tini "tini"}"
+      "--"
+    ];
+    Cmd = [
+      "${lib.getExe default}"
+    ];
+  };
+}