Add initContainer to wait for Postgres to be ready (#95)

* Add initContainer to wait for Postgres to be ready

- Adds an initContainer definition that waits until a given Postgres
  server is ready to accept connections using `pg_isready`
- Uses this initContainer in the Prefect migrations Job and the Prefect
  Server Deployment

This ensures that workloads depending on the database first check to see
if the database is ready. This should help avoid crash loop backoffs and
can, in certain cases, improve overall spin-up time.

Related to https://linear.app/prefect/issue/PLA-358/optimize-the-time-it-takes-for-the-prefect-operator-to-create-a-new

* Use smaller alpine flavor of postgresql image

internal/controller/prefectserver_controller.go

@@ -481,6 +481,9 @@ func (r *PrefectServerReconciler) postgresDeploymentSpec(server *prefectiov1.Pre
 				Labels: server.ServerLabels(),
 			},
 			Spec: corev1.PodSpec{
+				InitContainers: []corev1.Container{
+					r.initContainerWaitForPostgres(server),
+				},
 				Containers: []corev1.Container{
 					{
 						Name: "prefect-server",
@@ -526,6 +529,9 @@ func (r *PrefectServerReconciler) postgresMigrationJob(server *prefectiov1.Prefe
 				Labels: server.ServerLabels(),
 			},
 			Spec: corev1.PodSpec{
+				InitContainers: []corev1.Container{
+					r.initContainerWaitForPostgres(server),
+				},
 				Containers: []corev1.Container{
 					{
 						Name:    "prefect-server-migration",
@@ -581,6 +587,23 @@ func (r *PrefectServerReconciler) prefectServerService(server *prefectiov1.Prefe
 	return service
 }
 
+func (r *PrefectServerReconciler) initContainerWaitForPostgres(server *prefectiov1.PrefectServer) corev1.Container {
+	return corev1.Container{
+		Name:            "wait-for-database",
+		Image:           "postgres:16-alpine",
+		ImagePullPolicy: corev1.PullIfNotPresent,
+		Command: []string{
+			"/bin/sh",
+			"-c",
+			"until pg_isready -h $PREFECT_API_DATABASE_HOST -U $PREFECT_API_DATABASE_USER; do echo 'Waiting for PostgreSQL...'; sleep 2; done;",
+		},
+		Env: []corev1.EnvVar{
+			server.Spec.Postgres.HostEnvVar(),
+			server.Spec.Postgres.UserEnvVar(),
+		},
+	}
+}
+
 // SetupWithManager sets up the controller with the Manager.
 func (r *PrefectServerReconciler) SetupWithManager(mgr ctrl.Manager) error {
 	return ctrl.NewControllerManagedBy(mgr).