Alias /app /path/to/angular/build/dir
<Directory /path/to/angular/build/dir >
    RewriteEngine On
    RewriteBase /
    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteCond %{REQUEST_FILENAME} !-d
    RewriteRule . /a/index.html [L]

    AllowOverride All
    # Order allow,deny
    # allow from all
    Require all granted
</Directory>