`yarn start:contract` should auto-check chain blocks before starting

[amessbee]

Currently, we ask devs to use yarn docker:logs to manually check whether chain is up and running and producing blocks before running yarn start:contract. This should be automated. Here are two possible solutions:

1. At the end of scripts run by yarn start:docker, wait for a few (how many?) of blocks to be produced before exiting. I see that there is already

waitForBlock 2

in run-chain.sh. That's probably not enough?
2. At the start of yarn start:contract, wait for a few (how many?) of blocks to be produced before we start bundling/installing the contract.

A similar change should also be made to dapp-offer-up.