Rename .env.example -> defaults.env and make it the committed defaults layer.
Compose reads defaults.env then .env (later wins) via COMPOSE_ENV_FILES, feeding
BOTH container env and ${...} interpolation — so a changed default here reaches
every run without re-copying, and .env shrinks to just overrides + secrets.
No compose service restructure: the existing interpolated environment: blocks
already build the container env; the layering just supplies their values from two
files. (env_file: directive alone wouldn't work — it doesn't feed the image-tag /
cross-service Influx / ACME_EMAIL interpolation this compose relies on.)
Docs (DEPLOY §1, compose header, README) updated: export
COMPOSE_ENV_FILES=defaults.env,.env, keep only secrets/overrides in .env.
