feat(compose): add info on healthchecks and depends_on conditions

docs/docs/11_deploy_to_render/06_run_everything_docker_compose/README.md

@@ -12,7 +12,8 @@ services:
     ports:
       - "5000:80"
     depends_on:
-      - db
+      db:
+        condition: service_healthy
     env_file:
       - ./.env
     volumes:
@@ -24,6 +25,10 @@ services:
       - POSTGRES_DB=myapp
     volumes:
       - postgres_data:/var/lib/postgresql/data
+    healthcheck:
+      test: pg_isready -d $${POSTGRES_DB} -U postgres
+      interval: 2s
+      retries: 10
 volumes:
   postgres_data:
 ```
@@ -48,6 +53,12 @@ DATABASE_URL=postgresql://postgres:password@db/myapp
 When Docker Compose runs, it creates a virtual network[^1] which allows you to connect to `db`, which connects to the running `db` service container.
 :::
 
+In the `docker-compose.yml` file above you can also see that the `web` service depends on the `db` service, with the condition that it is healthy. A service is deemed "healthy" when its healthcheck passes.
+
+We've added a healthcheck to the `db` service which runs the `pg_isready`[^2] program using the supplied database and PostgreSQL user. This just tells us whether the PostgreSQL server is ready to respond to requests.
+
+Adding this means the `web` service won't start until the `db` service is ready to respond to requests.
+
 ## Named volumes in Docker Compose
 
 You'll notice that our `docker-compose.yml` file has these lines:
@@ -113,4 +124,5 @@ Note you must be in the folder that contains your `docker-compose.yml` file in o
 Running `docker compose down` will **not** delete your named volumes. You need to use the `-v` flag for that. Deleting the named volumes deletes the data in them irreversibly.
 :::
 
-[^1]: [Networking in Compose (official docs)](https://docs.docker.com/compose/networking/)
+[^1]: [Networking in Compose (official docs)](https://docs.docker.com/compose/networking/)
+[^2]: [pg_isready (PostgreSQL documentation)](https://www.postgresql.org/docs/current/app-pg-isready.html)