Add security headers to nginx config

Increase the security of the nginx server and the served page, by adding the following security headers to the nginx config:
- X-Frame-Options (Disables click jacking by disallowing the page to be run in a frame/iframe)
- X-XSS-Protection (Enables cross site scripting filtering)
- X-Content-Type-Options (Disables MIME sniffing and forces browser to use the type given in Content-Type.)
- Content-Security-Policy (Controls resources the user agent is allowed to load for a given page.)
- Referrer-Policy (Governs which referrer information sent in the Referer header should be included with requests made.)

Additional headers that could be added optionally:
- Strict-Transport-Security (Enforce HTTPS over HTTP)

config/docker/nginx.conf

@@ -17,6 +17,13 @@ http {
     server_name       localhost;
     index             index.html index.htm;
 
+    # Add security headers
+    add_header X-Frame-Options deny always;
+    add_header X-XSS-Protection "1; mode=block" always;
+    add_header X-Content-Type-Options nosniff always;
+    add_header Content-Security-Policy "default-src 'none'" always;
+    add_header Referrer-Policy strict-origin-when-cross-origin always;
+
     location / {
       alias            /usr/share/nginx/html/;