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.
