feat: add Docker support with environment configuration and entrypoint script

.env.example

@@ -0,0 +1,39 @@
+# EchoHub Server Configuration
+# Copy this file to .env and customize as needed:  cp .env.example .env
+# These override appsettings.json via ASP.NET Core's configuration hierarchy.
+
+# ── Server ───────────────────────────────────────────────────────────
+Server__Name=My EchoHub Server
+Server__Description=A self-hosted EchoHub chat server
+Server__PublicServer=false
+# Server__PublicHost=echohub.example.com
+# Server__Admins__0=adminUsername
+
+# ── JWT ──────────────────────────────────────────────────────────────
+# Auto-generated on first run if left empty. Only set if you need a stable secret across containers.
+# Jwt__Secret=
+# Jwt__Issuer=EchoHub.Server
+# Jwt__Audience=EchoHub.Client
+
+# ── Encryption ───────────────────────────────────────────────────────
+# Auto-generated on first run if left empty.
+# Encryption__Key=
+# Encryption__EncryptDatabase=false
+
+# ── Storage ──────────────────────────────────────────────────────────
+# Defaults are set in the Dockerfile to use /app/data for persistence.
+# Storage__CleanupIntervalHours=1
+# Storage__RetentionDays=30
+
+# ── IRC Gateway ──────────────────────────────────────────────────────
+Irc__Enabled=false
+# Irc__Port=6667
+# Irc__TlsEnabled=false
+# Irc__TlsPort=6697
+# Irc__TlsCertPath=
+# Irc__TlsCertPassword=
+# Irc__ServerName=echohub
+# Irc__Motd=Welcome to EchoHub IRC Gateway!
+
+# ── Logging ──────────────────────────────────────────────────────────
+# Serilog__MinimumLevel__Default=Information