feat: add Docker support with environment configuration and entrypoint script

src/EchoHub.Server/docker-entrypoint.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+
+# Persist appsettings.json in the data volume so auto-generated keys
+# (JWT secret, encryption key) survive container recreation.
+if [ ! -f /app/data/appsettings.json ]; then
+    cp /app/appsettings.example.json /app/data/appsettings.json
+fi
+ln -sf /app/data/appsettings.json /app/appsettings.json
+
+exec dotnet EchoHub.Server.dll