Add deprecation warnings.

Changes:

• Added deprecation warning. Container will still work until the next major OS release. See README.md for more details.

Update base to Ubuntu 20.04, Wine 7+.

Changes:

• base image migrated from 18.04 to 20.04.
• Remove Wine 5.x - <7.0 libfaudio0 patches. These are included in wine by defualt now.
• Reordered force_install_dir to be set before login, preventing execution halt with
  latest steamcmd release. See: ValveSoftware/steam-for-linux#8298

Add SDL libraries, ensure steam environment set.

• Add SDL libraries for steam install, preventing new SDL errors: ValveSoftware/steam-for-linux#7036
• Ensure steam user environment is setup correctly, updated instructions.
• Enforce set variables for startup scripts.
• Remove trace logging for startup scripts.
• Update examples.

Added 'experimental' tag.

Changes:

• Added 'experimental' tag which enables winehq-staging from WineHQ. This is the closest to HEAD without going into the development branch.
• Added package 'winbond' which fixes ntlm_auth not found errors for games that use it.
• Removed expose 27016 from default exposed list.

Include GDB, staging to /tmp, documentation updates.

Changes:

• GDB is included by default to allow for -debug option to be used in srcds_run.
• Staging directory moved to /tmp, removing build artifacts from repo.
• Documentation updated, including actual Windows/Linux server examples.
• Addressed Dockerfile line break continuation specification change.

Fix srcds_run crash with error message 'cat: hlds.{PID}.pid: No such file or directory'

Multiple causes:

• Pin to 18.04 not latest. Latest auto upgraded to Ubuntu 20.04. 20.04
  currently segfaults steamcmd.
• Targeted pruning of /var/lib/dpkg instead of wiping. amd64/i386 repos now
  enabled on Docker startup, enabling updates for both.
• Add explicit requirements for GDB and c++6; enabling -debug option
  for srcds_run which prevents PID error and auto reboot every 10 seconds.
  See: https://forums.alliedmods.net/showthread.php?t=300118
• wine 5.x added requirement of libfaudio0, but not included for version of
  Ubuntu < 19.10. Install these manually.

Changes:

• steam -> stable.
• winehq -> latest.