feat: docker image for ElectrumX, configured by environment

Replaces lukechilds/electrumx, unmaintained for years. ElectrumX 2.0.0 from the
upstream tag, rocksdb, built in a venv so only the runtime library follows into
the final image.

Every variable is ElectrumX's own — the entrypoint only fills defaults and
rejects the one mistake that is expensive to diagnose: NET spelled the way
bitcoind spells it ("test" for "testnet"), which otherwise fails deep inside a
coin-class lookup. DB_ENGINE defaults to rocksdb because 2.0 made the variable
required, and peer discovery is off because this image is for private indexers.

Verified on testnet2 against the sibling bitcoind image: coin BitcoinRegtest,
db height matching daemon height at 101, Electrum protocol answering on 50001.
That run also found what the README now states — 2.x refuses to serve unless
the daemon runs with both txindex=1 and txospenderindex=1.

docker/healthy_check.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+
+# Healthy = the server answers its own RPC. That happens once it has caught up
+# with the daemon and started serving, which is what callers care about: an
+# ElectrumX still building its index accepts no Electrum sessions at all.
+#
+# The RPC lives on localhost:8000 by default (SERVICES). Overriding SERVICES
+# without an rpc:// entry leaves nothing to ask — see README, "Notes".
+
+set -eu
+
+exec gosu electrumx electrumx_rpc getinfo > /dev/null