No/Wrong instructions on sh into the container on readme.md

# Overview
Following the [`readme.md`](https://github.com/coding-blocks/certificate-worker/tree/a018318c26a4cab2a097bb847de4ff1aba30a94f) on setting up the repo for development in my local machine, while `sh`-ing into the container using `$ docker exec -it /bin/sh` doesn't work. The main cause for this is `.env.sample` which on `yarn start` produces an error and the container exits immediately with error code 1. `The readme.md` needs to be updated with the necessary instructions to build, run and debug the docker container on local environments.