How to use environment variables in a Dockerfile?

To use environment variables in a Dockerfile, you can follow these steps:

1. Set the value of your environment variable using the ENV instruction in your Dockerfile.

ENV MY_VAR=my_value

Alternatively, you can use the ARG instruction to allow users to set the value at build time using the --build-arg flag.

ARG MY_VAR
ENV MY_VAR=${MY_VAR}

1. Use the environment variable in your Dockerfile by referencing it using the ${variable_name} syntax.

RUN echo "The value of MY_VAR is ${MY_VAR}"

1. Build your Docker image, either passing in the environment variable value using the --build-arg flag (if using ARG), or using the default value specified in the ENV instruction.

docker build --build-arg MY_VAR=custom_value -t myimage .

By using environment variables in your Dockerfile, you can make your images more flexible and easier to manage by allowing users to customize the behavior of the container at runtime. This can be especially useful for applications that require different settings in different environments or for handling sensitive information like passwords or keys.

Note that environment variables set in the Dockerfile can be viewed using the docker inspect command or by inspecting the image layers, so make sure to avoid including sensitive information in them. It’s also important to be consistent with variable names and uses across your Dockerfile, so that you can manage them more easily.