chore: add smtp environment variables to docker compose

.env.example

@@ -1,16 +1,16 @@
 # Read what every environment variable does: https://github.com/stonith404/pingvin-share#environment-variables
 
-# GENERAL
+# General
 APP_URL=http://localhost:3000
 SHOW_HOME_PAGE=true
 ALLOW_REGISTRATION=true
 ALLOW_UNAUTHENTICATED_SHARES=false
 MAX_FILE_SIZE=1000000000
 
-# SECURITY
+# Security
 JWT_SECRET=long-random-string
 
-# EMAIL
+# Email
 EMAIL_RECIPIENTS_ENABLED=false
 SMTP_HOST=smtp.example.com
 SMTP_PORT=587