Add Dockerfile and docker-compose.yml to run next to the Palladium node

ElectrumX is built from this source tree with RocksDB and joins the external
'palladium' network of the node in palladiumcore/docker, reaching it as
palladiumd:2332. RPC credentials come from .env (ignored by git, see
.env.example). The Bitcoin-specific contrib/Dockerfile is removed.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,8 @@
+.git
+.github
+tests
+docs
+contrib
+.env
+*.pyc
+__pycache__