Add wait_for.sh script

[poikilotherm]

I currently use this image for a bunch of things at https://github.com/IQSS/dataverse-kubernetes.

Many times, I use curl to check for a service to be ready. See example

Would it make sense for you to include a small shell script that capsules this in a file to make it more readable?

(It might also be interesting to include https://github.com/vishnubob/wait-for-it here, also I currently don't use it.)