[CoD2] Setup CoD2 with Docker

[Lonsofore]

Good news, everyone! Few updates:

1. Started to use semver in package versions
2. Added environment variables parsing. Now you could use COD2_SET_ prefix to set cvar! E.g. COD2_SET_net_port=28960. The same with COD2_SETA_ and COD2_SETS_, feel free to use. But don't use "_" and "=" in names and values. If you really need - use PARAMS env.
3. Now you don't have to specify CHECK_PORT - it would be parsed from envs or params string. But it you want, you still can - backwards compatibility!
4. Github Actions! Now all builds automatically upload to DockerHub and Github Packages, so don't worry about DockerHub limits.