fix: Copy bins and keys with mounts for ostree commit (#132)

I've been investigating more into how ostree works and how it relates to
running `ostree container commit` for each layer. I've decided to move
our pre-installed bins and public keys into their own stages and then
bind mount them into a `RUN` instruction so that we can just use `cp` to
get the files into the image and then call `ostree container commit`.
Now all of our layers in the image (after the base image) will be in the
ostree commit tree.

template/src/lib.rs

@@ -154,3 +154,10 @@ fn modules_exists() -> bool {
     let mod_path = Path::new("modules");
     mod_path.exists() && mod_path.is_dir()
 }
+
+mod filters {
+    #[allow(clippy::unnecessary_wraps)]
+    pub fn replace<T: std::fmt::Display>(input: T, from: char, to: &str) -> askama::Result<String> {
+        Ok(format!("{input}").replace(from, to))
+    }
+}