Docker entrypoint to test and wait for service dependencies

entrypoint.sh

```bash
#!/bin/bash
# List all deps using this format: ("host port", "host port"). Remember to link the container using the 'host' name.
requirements=("postgis 5432" "redis 6379" "elasticsearch 9200")

cmd="$@"

missing_requirement=true
while [ "${missing_requirement}" == "true" ]
do 
    missing_requirement=false
    sleep 1
    for req in "${requirements[@]}"
    do 
        if ! nc -z $req
        then
            echo "Waiting for ${req} to be up"
            missing_requirement=true
        else
            echo "${req} is up!"
        fi
    done
done

>&2 echo "All required services are up. Let's move on..!"

exec $cmd
```