staking-miner: docker images (#3682)

* staking-miner: docker image

* renaming

* add doc

* fix readme

* remove the creation of home for the polkadot user

* new builder image for the staking-miner

* remove mentions of the polkadot binary and add env

* foc: fix doc

* fix typos and dictionnary

* doc + sync up the 2 images

* doc: fine tuning the doc

* doc: security warning