[BUG] STARTTLS=true in .env is ignored

Setting STARTTLS=true in /opt/misp/.env is ignored.

Updating docker-compose.yml so it applies relevant variables from .env fixed this in my testing.

  web:
    <snip>
    environment:
      - SMTP_STARTTLS=${SMTP_STARTTLS:-true}
      - <other useful variables>

There may be other environment variables that the web container needs.