Docker: keep data in ~/.plm-electrumx (bind mount) instead of named volumes

db/ is the database, ssl/ the certificate; ELECTRUMX_DATADIR, PLM_UID and
PLM_GID configure it like the node's ~/.palladium. The entrypoint fails with a
clear message if a data directory is not writable.

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

.env.example

@@ -6,7 +6,13 @@ PLM_RPC_PASSWORD=change-me
 #REPORT_SERVICES=tcp://electrum.example.com:50001
 #ELECTRUMX_TCP_PORT=50001
 
-# SSL (port 50002): on by default, self-signed certificate generated once and kept in the electrumx-ssl volume.
+# Data directory of the host: db/ (database) and ssl/ (certificate). Create it first: mkdir -p ~/.plm-electrumx/db ~/.plm-electrumx/ssl
+#ELECTRUMX_DATADIR=~/.plm-electrumx
+# owner of the data directory, if it is not user 1000
+#PLM_UID=1000
+#PLM_GID=1000
+
+# SSL (port 50002): on by default, self-signed certificate generated once and kept in ssl/ of the data directory.
 #SSL_ENABLED=true
 # name (or IP) of the server, written in the generated certificate
 #SSL_HOSTNAME=electrum.example.com