Templates: fix the content type of templates

Templating is weird af in Rocket. Look into 3a541ae for more.
The content-type of the rendering is determined by the extension of the
template name.

But renaming these templates would break building the project for
development because it finds a phony template there. So the trick is to
default the `template_dir` to `args.upload` because it should never
interfere and will be always present.
This also fixes the hax in Dockerfile by making it the default.

Signed-off-by: Gunwant Jain <mail@wantguns.dev>

src/routes/index.rs

@@ -6,5 +6,5 @@ use std::collections::HashMap;
 pub async fn index() -> Option<Template> {
     let mut map = HashMap::new();
     map.insert("title", "bin");
-    Some(Template::render("index", &map))
+    Some(Template::render("index.html", &map))
 }