feat: docker image for Bitcoin Core, configured by environment

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.

docker/healthy_check.sh

@@ -0,0 +1,15 @@
+#!/bin/sh
+
+# Healthy = the RPC answers. Not "the chain is synced": an initial block
+# download takes days on mainnet, and a container that reports unhealthy for all
+# of it would be restarted by every orchestrator that watches health.
+#
+# The credentials come from cli.conf, written by run.sh, so they stay out of the
+# process list -- unlike the daemon's own arguments, which cannot avoid them.
+
+set -eu
+
+exec gosu bitcoin bitcoin-cli \
+	-datadir="${BITCOIN_DATA:-/data/.bitcoin}" \
+	-conf="${BITCOIN_DATA:-/data/.bitcoin}/cli.conf" \
+	getblockchaininfo > /dev/null