This is all great news!
A small complaint: <a href="http://textboard.org">?</a> in the navbar would probably be better off being something like <a href="/">return</a>. It might be nice to include a default barebones /static/index.html too.
Besides that, I might go down the hole of building nginx manually in Docker instead of using the stock image so that I can build it with ngx_fancyindex_module and ngx_http_substitutions_filter_module. (Are there other modules you're using?) Maybe also shove certbot in there for TLS while at it.