Refator stunnel and docs; merge environment variables

[envoyr]

This pull request refactors the SSL termination, the documentation and how we handle environment variables.

SSL termination is disabled by default, but can be enabled if either FROXLOR_OCTANE_HTTPS or FROXLOR_FORCE_HTTPS is set to true.

The container rebuilds the .env from three sources:

• .env.example
• existing .env
• current container environment variables starting with FROXLOR_*

Then it:

• removes the FROXLOR_ prefix
• keeps only valid KEY=value lines
• lets later values override earlier ones
• sorts the final variables
• writes them back to .env

The goal was to have an actual .env file, even though we modify it with the container environment variables.

Testing:

• Tested with an empty volume
• Tested with an existing volume
• Tested with container environment variables and existing .env
• Tested that .env.example values are used as defaults
• Tested that existing .env values override .env.example
• Tested that FROXLOR_* environment variables override both .env.example and existing .env
• Tested that non-FROXLOR_* environment variables are ignored
• Tested that invalid lines are ignored or removed
• Tested that the FROXLOR_ prefix is removed correctly
• Tested that final .env entries are sorted
• Tested that duplicate keys resolve to the latest value
• Tested SSL termination disabled by default
• Tested SSL termination enabled with FROXLOR_OCTANE_HTTPS=true
• Tested SSL termination enabled with FROXLOR_FORCE_HTTPS=true