Add Configure Different Host And Container Ports as a Docker TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1375 TILs and counting..._
+_1376 TILs and counting..._
 
 ---
 
@@ -24,6 +24,7 @@ _1375 TILs and counting..._
 * [CSS](#css)
 * [Deno](#deno)
 * [Devops](#devops)
+* [Docker](#docker)
 * [Elixir](#elixir)
 * [Gatsby](#gatsby)
 * [Git](#git)
@@ -184,6 +185,10 @@ _1375 TILs and counting..._
 - [SSL Certificates Can Cover Multiple Domains](devops/ssl-certificates-can-cover-multiple-domains.md)
 - [Wipe A Heroku Postgres Database](devops/wipe-a-heroku-postgres-database.md)
 
+### Docker
+
+- [Configure Different Host And Container Ports](docker/configure-different-host-and-container-ports.md)
+
 ### Elixir
 
 - [All Values For A Key In A Keyword List](elixir/all-values-for-a-key-in-a-keyword-list.md)

docker/configure-different-host-and-container-ports.md

@@ -0,0 +1,34 @@
+# Configure Different Host And Container Ports
+
+A `docker-compose.yml` file that sets up something like a PostgreSQL service
+will proxy a port from your host machine to a port on the docker container.
+
+A basic PostgreSQL service will look like this tying `5432` to `5432` under the
+`ports` section.
+
+```yaml
+version: "3.7"
+services:
+  postgres:
+    image: postgres:latest
+    restart: always
+    environment:
+      - POSTGRES_USER=postgres
+      - POSTGRES_PASSWORD=postgres
+      - POSTGRES_DB=postgres
+    ports:
+      - "5432:5432"
+    volumes:
+      - ./postgres-data:/var/lib/postgresql/data
+```
+
+Requests like queries from a `psql` instance that we send to `localhost:5432`
+will be proxied to `docker-container:5432`.
+
+Since those numbers are the same on both sides, it's not necessarily clear
+which is which. The left is the _host_ and the right is the _container_ --
+`[host-port]:[container-port]`. 
+
+If you need to use a port other than 5432 on your host machine (e.g. maybe
+you're running multiple Postgres servers at once), then you can just change the
+port number on the left side. How about `9876:5432`.