Is your feature request related to a problem? Please describe.
When running LND and bitcoind in containers (Podman/K8s), there’s no supported way for LND to read RPC credentials from files (i.e., mounted secrets).

This causes two problems in containerized deployments:

Security boundary: Giving LND read access to Bitcoin's datadir (for the cookie) or parsing bitcoin.conf which breaks security boundaries. i.e. LND's access to another apps data and conf directories.

Secrets handling: Putting plaintext user/pass in lnd.conf is undesirable; cookies also rotate on bitcoind restart, forcing coordination or restart of LND.

Describe the solution you'd like
Credential file flags in lnd.conf.
bitcoind.rpcuserfile=/run/secrets/bitcoin-rpc-user
bitcoind.rpcpassfile=/run/secrets/bitcoin-rpc-password

LND reads file contents at startup (and ideally on SIGHUP).

Describe alternatives you've considered
Wrapper entrypoint that reads secrets and execs lnd --bitcoind.rpcuser=... --bitcoind.rpcpass=.... Works but is operationally clunky and non-declarative.
Plaintext in lnd.conf for bitcoind.rpcuser/pass. Simple but violates standard secrets hygiene.
Cookie file via bitcoind.rpccookie. Keeps plaintext out of config but requires LND to read Core’s files and handle cookie rotation.

Additional context
With bitcoin configured using rpcauth=:,ドル clients still authenticate with username + plaintext password that matches that rpcauth line. A file-based credential mechanism lets LND consume those values securely from a secret without leaking them into lnd.conf. The public LND docs enumerate bitcoind.rpccookie, bitcoind.rpcuser, and bitcoind.rpcpass; adding file-based variants would align with container best practices.