Obviously the question is quite old but doing this is possible if you're willing to reconsider your chosen web server. Obviously you already have a Windows Server 2012R2 server so, in order to reduce complexity, there's no reason why you can't use IIS 8.5 to host your https site. I actually use this myself along side an SSTP VPN but I've also given you another option beneath the screenshot.
Once you have set the site up in IIS, create an https binding on the site to use a host header (Host name:) appropriate to the domain name of the site. This is how IIS knows which site to send the request to - yes, this means you can have multiple SSL websites on your IIS server too. I have a single certificate with multiple subject alternative names (SAN cert) that I use for both my SSTP VPN and the websites hosted on my IIS server, though you should be able to use separate certificates if you so wish.
You should also investigate Server Name Indication which is supported by IIS, though not enabled on my server.
If you desperately need to keep your Ubuntu box, install the Web Application Proxy role (sub-role of the Remote Access role) on to the Windows Server 2012R2 server (this role is new in 2012 R2). Configure that using the Remote Access Management Console (not RRAS console). Publish a web application using pass-through, define an external URL (the one external users will type in and resolve to your external IP and then an internal URL (that the Web Application Proxy server can resolve to your Ubuntu server's internal IP (HOSTS file works fine!). Use the SSL cert that you would have put on your Ubuntu box where it asks for a certificate. You can choose to not bother with defining SSL on your Ubuntu box and essentially let the Web Application Proxy service do SSL offloading. One thing to bear in mind is that your web server logs will show the WAP server's IP, not the user's external IP.
Two options there. Personally, I'd go with a single IIS 8.5 server to minimise estate sprawl :)