Talking to a PostgreSQL service container from inside a Docker container

Author: simonw

github-actions/service-containers-docker.md

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+# 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](https://docs.github.com/en/actions/guides/creating-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:
+
+```yaml
+name: Build, test and deploy
+
+on:
+  push:
+
+jobs:
+  build_test_deploy:
+    runs-on: ubuntu-latest
+    services:
+      postgres:
+        image: postgres
+        env:
+          POSTGRES_USER: postgres
+          POSTGRES_DB: postgres
+          POSTGRES_PASSWORD: postgres
+        ports:
+          - 5432:5432
+        # Health checks to wait until postgres has started
+        options: >-
+          --health-cmd pg_isready
+          --health-interval 10s
+          --health-timeout 5s
+          --health-retries 5
+    steps:
+    - uses: actions/checkout@v2
+    - name: Build the tagged Docker image
+      run: |-
+        docker build -t my-tag .
+    - name: Run tests
+      run: |-
+        docker run \
+          -e DATABASE_URL="postgres://postgres:[email protected]:5432/postgres" \
+          --entrypoint=/app/github-actions-runtests.sh \
+          my-tag
+```
+My `github-actions-runtests.sh` file uses [django-pytest](https://pytest-django.readthedocs.io/) and looks like this:
+```bash
+#!/bin/bash
+cd /app
+pytest --ds=config.test_settings
+```