Improvement: Allow flags on docker build

[goncer]

It took me a while to tune the docker file to add the param --feature=fast-runtime as suggested here: https://substrate.stackexchange.com/questions/957/speed-up-kusama-local-onboarding

I created this s diff. Would be awesome that docker builds to allow such parametrization for any user accept_arg.txt Thanks!

[bkchr]

Can you just create a pr?

[goncer]

done! https://github.com/paritytech/polkadot/pull/5285