How to setup a simple Apache2 Proxy to Mattermost?

Thanks for your help @coreyhulen but that gives me an error in my log files:

AH01144: No protocol handler was valid for the URL /api/v1/websocket. If you are using a DSO version of mod_proxy, make sure the proxy submodules are included in the configuration using LoadModule

and I get a blue bar error message a few seconds after loading up the instance:

"We cannot reach the Mattermost service. The service may be down or misconfigured. Please contact an administrator to make sure the WebSocket port is configured properly."

I have enabled tried to enable the proxy module like this:

$ sudo a2enmod proxy_http
Considering dependency proxy for proxy_http:
Module proxy already enabled
Module proxy_http already enabled

Here's my updated Apache VirtualHost file:

<VirtualHost *:443>
    ServerName mattermost.example.com
    ServerAlias mattermost.example.com

    DocumentRoot /var/www/mattermost

    SSLEngine on
    SSLCertificateFile /etc/apache2/ssl/server.pem
    SSLCertificateKeyFile /etc/apache2/ssl/server.key

    ProxyPreserveHost On

    RewriteEngine On

    RewriteCond %{REQUEST_URI}  ^/api/v1/websocket    [NC,OR]
    RewriteCond %{HTTP:UPGRADE} ^WebSocket$           [NC,OR]
    RewriteCond %{HTTP:CONNECTION} ^Upgrade$          [NC]
    RewriteRule .* ws://127.0.0.1:8065%{REQUEST_URI}  [P,QSA,L]

    RewriteCond %{DOCUMENT_ROOT}/%{REQUEST_FILENAME} !-f
    RewriteRule .* http://127.0.0.1:8065%{REQUEST_URI} [P,QSA,L]
    RequestHeader set X-Forwarded-Proto "https"

    <Location /api/v1/websocket>
        Require all granted
        ProxyPassReverse ws://127.0.0.1:8065/api/vi/websocket
        ProxyPassReverseCookieDomain 127.0.0.1 mattermost.example.com
    </Location>

    <Location />
        Require all granted
        ProxyPassReverse http://127.0.0.1:8065/
        ProxyPassReverseCookieDomain 127.0.0.1 mattermost.example.com
    </Location>
</VirtualHost>