In Docker, environment variables can be set in different ways, both during the image build process and when running the container instance. It's important to differentiate between build-time environment variables, runtime environment variables inside the container, and passing environment variables to the container instance.

Here's an explanation to help clarify the differences:

1. Build-time Environment Variables:
   • Build-time environment variables are used during the image build process to specify settings or configurations needed during the build.
   • These environment variables are only available during the build process and are not stored in the final image.
   • They are commonly used to adjust the behavior of certain commands in the Dockerfile.
   • Build-time environment variables are defined using the --build-arg flag during the docker build command.

Example in Dockerfile:

Example during build:

2. Run-time Environment Variables Inside the Container:
   • Run-time environment variables are set within the container when it is started.
   • These environment variables are available to the application running inside the container.
   • They can be specified in the Dockerfile using the ENV instruction or set when running the container using the -e flag.

Example in Dockerfile:

Example when running the container:

3. Passing Environment Variables to the Container Instance:
   • When running a Docker container, you can pass environment variables using the -e flag followed by the variable name and value.
   • These environment variables override any settings specified in the Dockerfile or inherited during the build process.
   • It allows you to customize the container's behavior without modifying the image.

Example when running the container:

By understanding and utilizing these options effectively, you can manage environment variables in Docker images and containers according to your specific requirements during both the build and run processes.