Remove default ENV to allow user provide environment variables via env files

[ileodo]

NextJS app allow user define environment variables by providing a .env.local file in /app/.

Those related lines in the Dockerfile are setting values to the environment variables, which will be passed in as process.env, which ranked as the top one in the loading order

This PR is removing these default variables so that user can just mount a .env.local file to set environment variable, which can avoid include the sensitive information in the docker-compose file.

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[kaaanata]

@ileodo I'm assuming you are going to mount .env.local file inside the docker container so that NextJS would load the env by default?

[ileodo]

yep.

[ileodo]

any chance to get an approval on this? thx