[APP-4023] Add shell flag

[ben-cutler-datarobot]

We have had issues before where an app build should fail (because a package wasn't available) but the build is marked as passing and the app fails to start. This can lead to confusion.
More broadly any script with the format:

<passing commands>
<failing command>
<passing commands>

will have a successful exit code and lead the app developer to think their build was a success. Commands like:

<passing commands>
<failing command>

will mark the build as failed. This is an implementation detail of shell specifically, and not every build-app will use shell. We can easily imagine a build-app with:

#!/usr/bin/env python
import ... 
if __name__=='__main__':
    do_stuff()

So, the smallest change we can do to remind users about this caveat is to just have it in the examples.

[chelsealiusa]

Is this react-base-app repo only used in custom app, not foundational app?
Will this also help reduce cost for the canary test run if something is wrong with the build?

[carsongee]

Why not the full "ye olde" set -euo pipefail?