updating to ubuntu 20.4 and necessary dockerfile changes

[jagohr]

Spent a few hours getting this running for me. The changes I had to make:

• Update the LinuxGSM index for vhserver
  • This apparently can change? The correct index seemed to be 121. There's probably a more reliable way to select the correct game server, but oh well.
• Update to Ubuntu 20.04
  • Starting vhserver was throwing runtime errors. The latest linuxgsm container is built on ubuntu 20.04 so I repurposed that
• Add missing dependencies for PlayFab (crossplay)
  • Found a few blog posts indicating these dependencies are required. Verified crossplay works on Xbox, and the server shows in the community server tab.

   libpulse0 \
   libpulse-dev \
   libc6-dev \
   libsdl2-2.0-0:i386 \
  

• resized the ACI container group to 1 cpu and 4 gb ram.
  • This seems to be right-sized from looking at the azure monitors, and should be ~30/month in azure costs