add custom nginx configuration

add custom nginx config so we can set custom 404 page

app-nginx.conf.sigil

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+worker_processes 1;
+error_log stderr;
+pid nginx.pid;
+daemon off;
+
+events {
+  worker_connections 768;
+}
+
+http {
+  types_hash_max_size 2048;
+  include mime.types;
+  server {
+    listen {{ $.PORT }};
+    server_name  _;
+    {{ if ne $.NGINX_ROOT "" }}
+      root /app/www/{{ $.NGINX_ROOT }};
+    {{ else }}
+      root /app/www;
+    {{ end }}
+    index index.html;
+    port_in_redirect off;
+
+    location / {
+      try_files $uri $uri/ /index.html;
+    }
+
+	error_page 404 /404.html;
+	location = /404.html {
+      root /app/www/{{ $.NGINX_ROOT }};
+	  internal;
+	}
+  }
+}

layouts/404.html

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+{{ define "content" }}
+  {{ partial "404.html" . }}
+{{ end }}

layouts/partials/404.html

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+<section class="container centered">
+  <div class="error">
+    <img src="/images/undraw-404.svg" alt="404 Page not found image">
+    <h1>{{ i18n "page_not_found" }}</h1>
+    <p>{{ i18n "page_does_not_exist" }}<br />{{ i18n "head_back" .Site.BaseURL | safeHTML }}</p>
+  </div>
+</section>