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

I like using Postgres.app 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:

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docker run --env DATABASE_URL="postgres://docker:docker-password@host.docker.internal:5432/pillarpointstewards" \
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   -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:

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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:

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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.