🐋 Bump versions in Dockerfile

[danstiner]

Bumps default versions in docker. Local testing against #60 shows a potential reduction in grey frames.

Looked through the changelogs and found the following two changes that looked relevant:

• janus - they disabled NACK buffer cleanup on outgoing keyframe by default [Make NACK buffer cleanup on outgoing keyframe configurable (see #2401) meetecho/janus-gateway#2402]
  • this looks related to our grey frames problem possibly, the linked discussion gets into more detail on why and has some interesting thoughts around how sending a keyframe more than once every ~five seconds can actually make the impact of packet loss worse
• libnice - slightly more efficient from using sendmmsg if possible to send multiple packets in one call

Also includes a change to install and use g++ version 10, I noticed that was needed to get #60 to build using docker.

Tested against #60 fairly extensively.

Full changelogs:

• https://github.com/meetecho/janus-gateway/blob/master/CHANGELOG.md
• https://github.com/libnice/libnice/blob/0.1.18/NEWS