What is an environment variable in a Docker container?

An environment variable in a Docker container serves as a setting that configures the behavior of the container. These variables allow you to customize the configuration of applications running inside the container without changing the application code.

For instance, you can set environment variables to specify database connection strings, API keys, or application modes (like development or production). By defining these variables when you create and run the container, you enable the same image to run in different environments with different configurations.

This flexibility is particularly valuable in scenarios where you want to deploy the same application across various environments without modifying the underlying image. The application inside the container can access these environment variables, adapting its runtime behavior based on the values provided.

Other listed options describe different concepts that are not applicable to environment variables specifically. For example, while secret keys are important for security, they refer to a specific use case rather than the general function of an environment variable. Similarly, container networking is a broader aspect of Docker's functionality, not exclusively defined by environment variables. Lastly, visual representation of data does not relate to the concept of environment variables, as they are not visual elements but rather key-value pairs stored in the container's environment.