nginx: redirect 404 to /404.html

Dockerfile

@@ -23,6 +23,6 @@ RUN sed -i -e 's/^worker_processes  auto;/worker_processes auto;/'\
  -e "/^events {$/ a \\\tmulti_accept on;\n\tuse epoll;"\
  -e "/^http {$/ a \\\tserver_tokens off;\n\tetag off;\n"\
  -e 's/#tcp_nopush/tcp_nopush/'\
- -e "/tcp_nopush/ a \\\ttcp_nodelay on;"\
+ -e "/tcp_nopush/ a \\\ttcp_nodelay on;\n\terror_page 404 /404.html;"\
  -e "s/^    */$(printf '\t')/"\
  /etc/nginx/nginx.conf