update to sha256 for checksum verification

[perobertson]

This is a rework of #404 which switches from sha1 to sha256.

This also drops the usage of the jinja templating in favour of the docker build-args.

• https://github.com/factoriotools/factorio-docker/pull/382#issuecomment-911659518

[Fank]

Required changes in the watchdog too, but they are will be pushed once this is merged