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>

templates/index.html.tera

@@ -1,4 +1,4 @@
-{% extends "base" %}
+{% extends "base.html" %}
 
 {% block styles %}
 <link rel="stylesheet" href="/static/css/index.css">

templates/pretty.html.tera

@@ -1,4 +1,4 @@
-{% extends "base" %}
+{% extends "base.html" %}
 
 {% block styles %}
 <style>