Replaces lncm/bitcoind, which pins Core 26 — a version that still has legacy
wallets, while the code that talks to it (bt's BitcoindClient) is written for
the descriptor-only behaviour of 29 and later.
The binaries are the official release build, verified by SHA-256 in the same
layer that downloads them; a version bump that forgets the checksum fails the
build instead of shipping something unverified. uid 1000 and /data/.bitcoin are
kept from the image this replaces, so an existing named volume survives the
switch without a recursive chown of a synced chain.
Verified on testnet2: regtest node healthy in ~12 s, descriptor wallet, 101
blocks mined, sendtoaddress accepted — the last one being the check for
BITCOIND_FALLBACKFEE, without which a fresh chain refuses to send.
