Add missing environment variable to Coturn service

[omidmaldar]

What

This pr adds ENABLE_HTTPS_PROXY environment variable to the Coturn service. This var is required to be present in the container and will be used in /mod/coturn/entrypoint.sh for extracting TLS certificates. This is a boolean var which reflects user decision regarding enabling an automatic http proxy.

When var is not available

• Looking at docker logs one can read the following error when the var is not present but user wishes to enable an http proxy

  ERROR: certificate not found, but coturn relies on it.
  Use either auto HTTPS proxy or
  provide path to certificates in .env file

• The bbb-docker_coturn_1 container restarts for ever

  
  5d2d44c70109   instrumentisto/coturn:4.5   "docker-entrypoint.s…"   About a minute ago   Restarting (1) 28 seconds ago                              bbb-docker_coturn_1

[alangecker]

unfortunately @clandmeter was with #146 already quicker resolving this, but thanks for the elaborate, exemplary PR! :)

[omidmaldar]

Thanks and no worries. I should have checked the list of open PRs before suggesting this change.