build (docker): variable DUNITER_CHAIN_NAME

This variable will configure the `--chain` substrate option. This way
we can configure `--chain dev`, `--chain gtest`, or `--chain g1` with
only one variable.

DUNITER_CHAIN_NAME=dev replaces DUNITER_DEVELOPMENT_MODE=true as `--dev`
implies `--chain dev`. The only difference is that `--dev` disables
RPC CORS. Then the entrypoint automatically sets `--dev` only when
DUNITER_CHAIN_NAME=dev. In this case `--chain dev` is not used because
substrate doesn't want both `--dev` and `--chain dev`.

dev