Attempt to run unit tests in github actions

Airflow needs the db to be intialised even if DAGS aren't actually run.

This github actions workflow creates a postgres service container[1],
and then runs initdb before running pytest.

However, I can't use act to test this locally, because it doesn't
support service containers[2].

[1]. https://docs.github.com/en/free-pro-team@latest/actions/guides/creating-postgresql-service-containers
[2]. nektos/act#173

.github/workflows/ci.yaml

@@ -5,17 +5,59 @@ on: [push]
 jobs:
   build:
     runs-on: ubuntu-latest
+    container: python:3.8.7-slim
+
+    services:
+      postgres:
+        image: postgres:10.8
+        env:
+          POSTGRES_USER: postgres
+          POSTGRES_PASSWORD: postgres
+          POSTGRES_DB: postgres
+        ports:
+        - 5432:5432
+        # needed because the postgres container does not provide a healthcheck
+        options: --health-cmd pg_isready --health-interval 10s --health-timeout 5s --health-retries 5
 
     steps:
     - uses: actions/checkout@v2
-    - name: Set up Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v2
-      with:
-        python-version: 3.8.6
+
+    - name: Install postgresql-client package
+      run: |
+        apt-get update
+        apt-get install --yes --no-install-recommends postgresql-client
+
+    - name: Create database
+      run: |
+        createdb airflow
+      env:
+        PGUSER: postgres
+        PGPASSWORD: postgres
+        PGHOST: postgres
+
     - name: Install dependencies
       run: |
         pip --version
+        pip install wheel
         pip install -r requirements.txt --use-deprecated=legacy-resolver
+
+    - name: initdb
+      env:
+        AIRFLOW__CORE__UNIT_TEST_MODE: True
+        AIRFLOW__CORE__DAGS_FOLDER: src/dags
+        AIRFLOW__CORE__SQL_ALCHEMY_CONN: postgres://postgres:postgres@postgres/airflow
+        AIRFLOW__CORE__EXECUTOR: LocalExecutor
+      run: |
+        airflow initdb
+
     - name: Test with pytest
       run: |
         pytest
+
+      env:
+        AIRFLOW__CORE__UNIT_TEST_MODE: True
+        AIRFLOW__CORE__DAGS_FOLDER: src/dags
+        AIRFLOW__CORE__SQL_ALCHEMY_CONN: postgres://postgres:postgres@postgres/airflow
+        AIRFLOW__CORE__EXECUTOR: LocalExecutor
+
+