Hardcoded ENV values can be a tedious solution if they need to be changed often. Imagine having to edit your Dockerfile again and again with each build.

You can do better! Introduce a new ARG variable, and reference it to set your initial ENV variables dynamically during the build:

ARG var_name # you could give this a default value as well
ENV env_var_name=$var_name

the env_var_name environment variable value passed with var_name will be available to future containers. If needed, you can override it when starting up a container from the image.

They are merely default values after all.