Kubernetes deployment of a php test application
If you do a kubectl describe pod <YOUR DB POD> you'll see a bit more informations on why the pod is crashing.
If that's not enough, try kubectl logs <YOUR DB POD>, you'll have logs with all errors and warning.
You should check on what IP is your minikube configured and use it instead of localhost.
This can be checked in dashboard following Cluster > Nodes > minikube path, or using this command minikube ip and this should be used to check if service is working.
I also strongly advice to check Set up Ingress on Minikube with the NGINX Ingress Controller.