Per standard Docker convention. The active `.env` is per-host
(contains the actual TSIG secret + any host-specific port/hostname
overrides). The `.env.example` template documents the expected
variables with stub values so a fresh checkout knows what to copy.
Also: docker-compose.yml now passes ACME_TSIG_SECRET to the coredns
container via plain `environment:` directive -- compose auto-reads
`.env` for substitution. No --env-file gymnastics needed at the
invocation level.
