Allowing a container in Docker Desktop for Mac to talk to a PostgreSQL server on the host machine

Author: simonw

docker/docker-for-mac-container-to-postgresql-on-host.md

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+# Allowing a container in Docker Desktop for Mac to talk to a PostgreSQL server on the host machine
+
+I like using [Postgres.app](https://postgresapp.com/) to run PostgreSQL on my macOS laptop. I use it for a bunch of different projects.
+
+When I deploy applications to Fly.io I build them as Docker containers and inject the Fly PostgreSQL database details as a `DATABASE_URL` environment variable.
+
+In order to test those containers on my laptop, I needed to figure out a way to set a `DATABASE_URL` that would point to the PostgreSQL I have running on my own laptop - so that I didn't need to spin up another PostgreSQL Docker container just for testing purposes.
+
+## host.docker.internal
+
+The first thing to know is that Docker for Desktop sets `host.docker.internal` as a magic hostname inside the container that refers back to the IP address of the host machine.
+
+So ideally something like this should work:
+
+    docker run --env DATABASE_URL="postgres://docker:[email protected]:5432/pillarpointstewards" \
+       -p 8080:8000 pillarpointstewards
+
+I'm using `-p 8080:8000` here to set port 8080 on my laptop to forward to the Django application server running on port 8000 inside the container.
+
+## Creating the account and granting permissions
+
+To create that PostgreSQL account with username `docker` and password `docker-password` (but pick a better password than that) I used Postico to open a connection to my `postgres` database and ran the following:
+
+    create role docker login password 'docker-password';
+
+Then I connected to my application database (in this case `pillarpointstewards`) and ran the following to grant permissions to that user:
+```sql
+GRANT ALL ON ALL TABLES IN SCHEMA "public" TO docker;
+```
+Having done this, the container run with the above `DATABASE_URL` environment variable was able to both connect to the server and run Django migrations too.