Fix: Docker Compose Example (#25546)

## Summary & Motivation

Docker Compose example deployment for Dagster throws errors on
initialisation because the webserver and daemon do not wait for the
postgreSQL database to be ready before attempting to connect.

Added healthcheck to postgresql container to fix this issue.

## How I Tested These Changes

Works on my machine and a new EC2 instance with relevant bits installed.

examples/deploy_docker/docker-compose.yml

@@ -2,7 +2,8 @@ version: "3.7"
 
 services:
   # This service runs the postgres DB used by dagster for run storage, schedule storage,
-  # and event log storage.
+  # and event log storage. Depending on the hardware you run this Compose on, you may be able
+  # to reduce the interval and timeout in the healthcheck to speed up your `docker-compose up` times.
   docker_example_postgresql:
     image: postgres:11
     container_name: docker_example_postgresql
@@ -12,6 +13,11 @@ services:
       POSTGRES_DB: "postgres_db"
     networks:
       - docker_example_network
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U postgres_user -d postgres_db"]
+      interval: 10s
+      timeout: 8s
+      retries: 5
 
   # This service runs the gRPC server that loads your user code, in both dagster-webserver
   # and dagster-daemon. By setting DAGSTER_CURRENT_IMAGE to its own image, we tell the
@@ -64,8 +70,10 @@ services:
     networks:
       - docker_example_network
     depends_on:
-      - docker_example_postgresql
-      - docker_example_user_code
+      docker_example_postgresql:
+        condition: service_healthy
+      docker_example_user_code:
+        condition: service_started
 
   # This service runs the dagster-daemon process, which is responsible for taking runs
   # off of the queue and launching them, as well as creating runs from schedules or sensors.
@@ -88,8 +96,10 @@ services:
     networks:
       - docker_example_network
     depends_on:
-      - docker_example_postgresql
-      - docker_example_user_code
+      docker_example_postgresql:
+        condition: service_healthy
+      docker_example_user_code:
+        condition: service_started
 
 networks:
   docker_example_network: