Create env_variables.md (#743)

Signed-off-by: Jael Gu <[email protected]>

Author: jaelgu

docs/user_guide/env_variables.md

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+# Milvus Backup: Environment Variables
+
+Milvus Backup supports environment variables for configuration and customization.
+These variables allow you to override settings defined in your configuration file or inject secrets and runtime options.
+
+## How to Use Environment Variables
+
+You can set environment variables in your shell before running Milvus Backup. For example:
+
+```bash
+$ export MILVUS_PORT=29530
+$ ./milvus-backup server
+```
+
+---
+
+## Supported Environment Variables
+
+> All variables are optional. If not set, defaults are used (see [`backup.yaml`](https://github.com/zilliztech/milvus-backup/blob/main/configs/backup.yaml)).
+>
+> For the latest list of supported environment variables, refer to [`paramtable/base_table.go`](https://github.com/zilliztech/milvus-backup/blob/main/core/paramtable/base_table.go) in the source code.
+
+- [Milvus Connection](#milvus-connection)
+- [MinIO (Object Storage)](#minio-object-storage)
+- [Logging](#logging)
+- [HTTP API](#http-api)
+- [Others](#others)
+
+### Milvus Connection
+
+| Config Key           | Env Variable        | Description                                 |
+|----------------------|---------------------|---------------------------------------------|
+| milvus.address       | MILVUS_HOST         | Milvus proxy address                        |
+| milvus.port          | MILVUS_PORT         | Milvus proxy port                           |
+| milvus.user          | MILVUS_USER         | Milvus user name                            |
+| milvus.password      | MILVUS_PASSWORD     | Milvus password                             |
+| milvus.caCertPath    | MILVUS_CA_CERT_PATH | TLS CA certificate path                     |
+| milvus.serverName    | MILVUS_SERVER_NAME  | Milvus server name (TLS)                    |
+| milvus.mtlsCertPath  | MILVUS_MTLS_CERT_PATH| Mutual TLS certificate path                |
+| milvus.mtlsKeyPath   | MILVUS_MTLS_KEY_PATH| Mutual TLS key path                         |
+
+### MinIO (Object Storage)
+
+| Config Key                  | Env Variable          | Description                                  |
+|-----------------------------|-----------------------|----------------------------------------------|
+| minio.address               | MINIO_ADDRESS         | MinIO server address                         |
+| minio.port                  | MINIO_PORT            | MinIO server port                            |
+| minio.accessKeyID           | MINIO_ACCESS_KEY      | MinIO access key ID                          |
+| minio.secretAccessKey       | MINIO_SECRET_KEY      | MinIO secret access key                      |
+| minio.useSSL                | MINIO_USE_SSL         | Use SSL for MinIO                            |
+| minio.bucketName            | MINIO_BUCKET_NAME     | MinIO bucket name                            |
+| minio.gcpCredentialJSON     | GCP_KEY_JSON          | Path to GCP credentials JSON for MinIO       |
+
+#### Backup-specific MinIO Variables
+
+| Config Key                        | Env Variable                  | Description                                  |
+|-----------------------------------|-------------------------------|----------------------------------------------|
+| minio.backupBucketName            | MINIO_BACKUP_BUCKET_NAME      | Backup bucket name                           |
+| minio.backupAddress               | MINIO_BACKUP_ADDRESS          | Backup MinIO server address                  |
+| minio.backupPort                  | MINIO_BACKUP_PORT             | Backup MinIO server port                     |
+| minio.backupAccessKeyID           | MINIO_BACKUP_ACCESS_KEY       | Backup MinIO access key ID                   |
+| minio.backupSecretAccessKey       | MINIO_BACKUP_SECRET_KEY       | Backup MinIO secret access key               |
+| minio.backupUseSSL                | MINIO_BACKUP_USE_SSL          | Use SSL for backup MinIO                     |
+
+### Logging
+
+| Config Key         | Env Variable     | Description                          |
+|--------------------|------------------|--------------------------------------|
+| log.level          | LOG_LEVEL        | Logging level (`info`, `debug`, etc) |
+| log.console        | LOG_CONSOLE      | Print logs to console                |
+| log.file.rootPath  | LOG_FILE_PATH    | Path to log file                     |
+
+### HTTP API
+
+| Config Key          | Env Variable          | Description                   |
+|---------------------|-----------------------|-------------------------------|
+| http.simpleResponse | HTTP_SIMPLE_RESPONSE  | Enable simple HTTP responses  |
+
+### Others
+
+| Config Key                  | Env Variable                  | Description                                   |
+|-----------------------------|-------------------------------|-----------------------------------------------|
+| backup.maxSegmentGroupSize  | BACKUP_MAX_SEGMENT_GROUP_SIZE | Maximum segment group size                    |
+| backup.parallelism.backupCollection | BACKUP_PARALLELISM_BACKUP_COLLECTION | Backup collection parallelism         |
+| backup.keepTempFiles        | BACKUP_KEEP_TEMP_FILES         | Keep temporary files after backup             |