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/pretty_retrieve.rs

@@ -16,7 +16,7 @@ pub async fn pretty_retrieve(id: PasteId<'_>) -> Option<Template> {
     let mut map = HashMap::new();
     map.insert("title", id.to_string());
     map.insert("body", contents);
-    let rendered = Template::render("pretty", &map);
+    let rendered = Template::render("pretty.html", &map);
 
     match tree_magic::match_filepath("text/plain", &filepath) {
         true => Some(rendered),