feat(compose): add new lecture on running commands in a container

docs/docs/04_docker_intro/05_run_commands_in_docker_containers/README.md

@@ -0,0 +1,24 @@
+# How to run commands inside a Docker container
+
+If you run your API using Docker Compose, with the `docker compose up` command, you may also want to be able to execute arbitrary shell commands in the container.
+
+For example, later on in the course we will look at database migrations.
+
+To execute a database migration, we need to run a specific command, `flask db mgirate`.
+
+If we use Docker Compose, we'll need to run the command inside the running container, and not in a local terminal.
+
+You can run any arbitrary command in a running container like so:
+
+```bash
+docker compose exec web flask db migrate
+```
+
+This command is split into 4 parts:
+
+- `docker compose`: uses the Docker Compose part of the Docker executable
+- `exec`: used to run a command in a specific Docker Compose service
+- `web`: which Docker Compose service to run the command in
+- `flask db migrate`: the command you want to run
+
+That's all! Just remember while following the course, that if I run any commands in my local terminal and you are using Docker Compose, you should precede the commands with `docker compose exec web`.

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:
@@ -24,14 +25,18 @@ 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:
 ```
 
 The `postgres` image accepts various environment variables, among them:
 
 - `POSTGRES_PASSWORD`, defaulting to `postgres`
-- `POSTGERS_DB`, defaulting to `postgres`
+- `POSTGRES_DB`, defaulting to `postgres`
 - `POSTGRES_USER`, defaulting to `postgres`
 - `POSTGRES_HOST`, defaulting to `localhost`
 - `POSTGRES_PORT`, defaulting to `5432`
@@ -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)