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>

docker-compose.yml

@@ -6,7 +6,7 @@ services:
     ports:
       - 127.0.0.1:6163:6163
     environment:
-      - BIN_PORT=6163
+      - BIN_PORT=6163 # Defaults to 6162
       - BIN_LIMITS={form="16 MiB"}
     volumes:
       - ./upload:/upload  # upload folder will have your pastes