simulators/ethereum/eest: fail image build with unknown `build-arg`

[danceratopz]

This will happily build, despite the typo in "branch":

cd simulators/ethereum/eest/consume-engine/
docker build --no-cache --build-arg bronch=future-fork .

Low prio, but it'd be nice to fail with unknown build-args

[Abiji-2020]

@danceratopz The docker-engine discards it automatically so that this will happily build..
If the value branch is mandatory we can have a check condition.