Add a new _faq_sourcing_when_container_starts

[VladimirFokow]

After the discussion #1854 I've added rough notes for the new FAQ answer: How do I source a file automatically when container starts?

• TODO: check that the link ch-run#environment-variables works

[reidpr]

Fantastic, thank you! Do you want us to take it from here, or are you planning more?

[VladimirFokow]

take it from here, that's everything I could add from my side😄