Use `gck.env` instead of `.env`

We try to solve the:

ERROR: In file ./.env: environment variable name 'export USE_WEBPACK_DEV' may not contain whitespace

The Compose 1.25 added --env-file, but I really want us to support 1.21 as this is the latest supported version by Debian/Buster.

This changes the env file to be gck.env. It is simple rename of file, and make will complain when loading it initially.