need env var check

[geremyCohen]

At https://ecsworkshop.com/nodejs/backend/#deploy-frontend-1 I ran into an issue while creating the service using the ecs-cli fargate mode path.

I had closed the terminal I used to setup Frontend, so I lost the vars I had set previously on the:

https://ecsworkshop.com/frontend/frontend/#deploy-frontend-1 > Set environment variables from our build section.

I ran the envsubst, and the yaml had empty fields. and as you would expect, the subsequent ecs-cli command failed.

So maybe some sort of check that the vars exist, saving them to the .bash_profile or just forcing the user to re-set the vars again would be good here.

[geremyCohen]

Any fix should also be applied on the crystal deployment.