docs: standalone env file precedence and with-docker-multi-env secrets (#101728)
Stacked on #101699.
## Summary
- Standalone env files docs: runtime environment variables take
precedence over the copied `.env` and `.env.production`, and unset
variables fall back to them. Values from `.env.local` reach build-time
work only. Secrets should be provided at runtime, as the `with-docker`
example does. Deleting the copied files also removes their fallback
values. Also corrects how the `'next-server'` exclude key is matched.
- `with-docker-multi-env`: the `.sample` files are copied to
`.env.production` and ship in the image. The example now keeps local env
files out of the build context, explains that the `.sample` files must
hold public values only, passes `API_SECRET` at runtime through
`compose.yaml` (with `pages/api/secret.js` reporting whether it is set),
and fixes dead docs links.
## Commits
- docs: describe standalone env file precedence and runtime secrets
- examples: keep secrets out of with-docker-multi-env images
## Follow-up
- Environment variables guide: say that `.env` files are for local
development, and that production secrets come from the deployment
environment or a secret store.
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