Docker: pass DONATION_ADDRESS to the server, document it

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

.env.example

@@ -4,6 +4,8 @@ PLM_RPC_USER=username
 PLM_RPC_PASSWORD=change-me
 #CACHE_MB=1200
 #REPORT_SERVICES=tcp://electrum.example.com:50001
+# address for donations, sent to the clients; not checked by the server, so double-check it
+#DONATION_ADDRESS=
 #ELECTRUMX_TCP_PORT=50001
 
 # Data directory of the host: db/ (database) and ssl/ (certificate). Create it first: mkdir -p ~/.plm-electrumx/db ~/.plm-electrumx/ssl