Paso 7: Habilitar redirección HTTP a HTTPS

La configuración para la redirección de HTTP a HTTPS se realiza en el archivo /etc/apache2/sites-available/bookstack.conf que queda de la siguiente manera:

<VirtualHost *:80>
  ServerName sergio-jimenez.net

  ServerAdmin webmaster@localhost
  DocumentRoot /var/www/bookstack/public/

  Redirect permanent / https://sergio-jimenez.net/

  <Directory /var/www/bookstack/public/>
      Options -Indexes +FollowSymLinks
      AllowOverride None
      Require all granted
      <IfModule mod_rewrite.c>
          <IfModule mod_negotiation.c>
              Options -MultiViews -Indexes
          </IfModule>

          RewriteEngine On

          # Handle Authorization Header
          RewriteCond %{HTTP:Authorization} .
          RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]

          # Redirect Trailing Slashes If Not A Folder...
          RewriteCond %{REQUEST_FILENAME} !-d
          RewriteCond %{REQUEST_URI} (.+)/$
          RewriteRule ^ %1 [L,R=301]

          # Handle Front Controller...
          RewriteCond %{REQUEST_FILENAME} !-d
          RewriteCond %{REQUEST_FILENAME} !-f
          RewriteRule ^ index.php [L]
      </IfModule>
  </Directory>

  ErrorLog ${APACHE_LOG_DIR}/error.log
  CustomLog ${APACHE_LOG_DIR}/access.log combined

RewriteCond %{SERVER_NAME} =sergio-jimenez.net
RewriteRule ^ https://%{SERVER_NAME}%{REQUEST_URI} [END,NE,R=permanent]
</VirtualHost>

La línea clave es:

Redirect permanent / https://sergio-jimenez.net/

Fuente: Redirect HTTP to HTTPS in Apache