Use shellchecker for bash scripts

Overview

At the moment we have a good amount of scripts that use bash to test/build/run the Runner in CI and locally. However, we don't follow best practice when it comes to writing bash, if we run shellcheck we see some errors reported.

Proposal

• Fix existing issues when you run shellcheck
• Create new Job that will run shellcheck on the scripts to make sure we don't introduce new issues.

/cc @pedropombeiro