Undefined MIX_ENV in Dockerfile

[am-nimrah]

The Dockerfile references the MIX_ENV argument without an explicit definition. To resolve this issue, consider adding the following line before the FROM statement:
ARG MIX_ENV=prod
This ensures that the MIX_ENV argument is defined, providing a default value of 'prod' unless explicitly overridden during the build.

[sheharyarn]

As mentioned in previous comments, the dockerfile files in the codebase are templates. In the UI, the user will see the MIX_ENV correctly defined and referenced in the rendered Dockerfile.