您如何在Docker容器中指定環境變量？

在Docker容器中設置環境變量有三種常見方式：使用-e標誌、在Dockerfile中定義ENV指令、或通過Docker Compose管理。 1. 使用docker run時添加-e標誌可直接傳入變量，適合臨時測試或CI/CD集成；2. 在Dockerfile中使用ENV設置默認值，適用於不常更改的固定變量，但不適合區分不同環境配置；3. Docker Compose可通過environment塊或.env文件定義變量，後者更利於開發協作和配置分離，並支持變量替換。根據項目需求選擇合適方法或組合使用多種方式。

You can specify environment variables in a Docker container in several practical ways, depending on your setup and needs. The most common methods include using the -e flag when running a container, defining them in a Dockerfile with ENV , or managing them via a .env file when using Docker Compose.

Using the -e flag with docker run

When you start a container manually using docker run , you can pass environment variables directly on the command line with the -e flag.

For example:

 docker run -d -e MY_VAR="hello" my-app

This sets the variable MY_VAR to "hello" inside the container.

• You can set multiple variables by repeating the -e flag.
• This is useful for one-off containers or testing different configurations.
• It's also helpful when integrating with CI/CD systems where secrets or settings are injected dynamically.

If you're setting sensitive data like API keys or passwords, be cautious about exposing them in shell history or logs.

Defining environment variables in a Dockerfile

You can also define default environment variables directly in your Dockerfile using the ENV instruction.

Example:

 ENV MY_VAR hello
ENV LOG_LEVEL debug

These values will be baked into the image and available in every container started from it unless overridden at runtime.

• This method is good for setting defaults that rarely change.
• It's not ideal if you need different values across environments (like dev vs prod).
• Keep in mind these values are visible in the image metadata, so avoid putting sensitive info here.

Managing environment variables with Docker Compose

When working with Docker Compose, you can define environment variables in two main ways:

1. Directly in the environment block of your docker-compose.yml :

    services:
     app:
       image: my-app
       environment:
         MY_VAR: "hello"

2. Using an external .env file with the env_file option:

    services:
     app:
       image: my-app
       env_file:
         - .env

The second approach helps keep your configuration clean and portable. The .env file looks like this:

 MY_VAR=hello
LOG_LEVEL=debug

• This is great for local development and team setups.
• Make sure to add .env files to .gitignore if they contain sensitive data.
• Docker Compose supports variable substitution too, so you can reference existing shell variables.

That's how you handle environment variables in Docker — through CLI flags, Dockerfiles, or compose files. Each method has its use case, and often you'll combine them depending on your project's complexity.

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