Talking to a PostgreSQL service container from inside a Docker container#

I have a Django application which uses PostgreSQL. I build the Django application into its own Docker container, push that built container to the GitHub package registery and then deploy that container to production.

I wanted to run the tests inside the container as part of the deployment process, to make sure the container that I build is ready to be deployed (via continuous deployment).

In production I’m using Digital Ocean PostgreSQL rather than running PostgreSQL in a container. For running the tests I decided to use GitHub’s PostgreSQL service containers to run the tests.

But how do you set it up so tests running inside a Docker container can talk to the PostgreSQL service container provided by the GitHub Actions environment?

This took a while to figure out. The key insight was that Docker containers (at least on Linux) have a magic IP address, 172.17.0.1, which can be used to access their host environment - and GitHub’s PostgreSQL container is available to that host environment on localhost port 5432.

So here’s the recipe I ended up using:

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name: Build, test and deploy
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on:
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  push:
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jobs:
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  build_test_deploy:
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    runs-on: ubuntu-latest
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    services:
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      postgres:
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        image: postgres
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        env:
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          POSTGRES_USER: postgres
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          POSTGRES_DB: postgres
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          POSTGRES_PASSWORD: postgres
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        ports:
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          - 5432:5432
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        # Health checks to wait until postgres has started
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        options: >-
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          --health-cmd pg_isready
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          --health-interval 10s
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          --health-timeout 5s
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          --health-retries 5
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    steps:
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    - uses: actions/checkout@v2
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    - name: Build the tagged Docker image
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      run: |-
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        docker build -t my-tag .
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    - name: Run tests
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      run: |-
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        docker run \
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          -e DATABASE_URL="postgres://postgres:postgres@172.17.0.1:5432/postgres" \
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          --entrypoint=/app/github-actions-runtests.sh \
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          my-tag

My github-actions-runtests.sh file uses django-pytest and looks like this:

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#!/bin/bash
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cd /app
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pytest --ds=config.test_settings