Add example .env

[ci skip]

.env.sample

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+TYPE=discourse
+
+DOMAIN=discourse.example.com
+## Domain aliases
+#EXTRA_DOMAINS=', `www.discourse.example.com`'
+LETS_ENCRYPT_ENV=production
+
+# Outgoing email
+#DISCOURSE_SMTP_HOST=
+#DISCOURSE_SMTP_PORT=
+#DISCOURSE_SMTP_USER=
+#DISCOURSE_SMTP_PROTOCOL=
+#DISCOURSE_SMTP_AUTH=
+
+# SMTP authentication
+#COMPOSE_FILE="compose.yml:compose.smtpauth.yml"
+#SECRET_SMTP_PASSWORD_VERSION=v1
+
+SECRET_DB_PASSWORD_VERSION=v1