Add instructions on how to launch directly from Docker

[cmichi]

Should be in the form of https://github.com/paritytech/cargo-contract#installation-using-docker-image.

The Docker which is referred there already contains the executable of the latest substrate-contracts-node release (this is done here), so we can just use this CI image here as well. I think the only that needs to be modified is to remove the folder sharing arguments and add some port forwarding ones.

ToDo

• [ ] Add the instructions to the readme in this repo.
• [ ] Add pointer to ink-docs where we describe how to launch subtrate-contracts-node.
• [ ] Add pointer to substrate-docs where we describe how to launch subtrate-contracts-node