docker-compose: add example test file

Add example docker-compose file and also use it as a test during the build
of each commit to make sure pg_back supports all major revisions of
Postgres.

.github/workflows/docker.yml

@@ -15,6 +15,25 @@ env:
   IMAGE_NAME: pg_back
 
 jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        pg_version: [12, 13, 14, 15]
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3.5.3
+
+      - name: Test Docker container
+        uses: adambirds/docker-compose-action@v1.3.0
+        env:
+          PG_VERSION: ${{ matrix.pg_version }}
+        with:
+          compose-file: ./docker/compose.test.yml
+          up-flags: --abort-on-container-exit --exit-code-from pg_back
+          down-flags: --volumes
+
   buildx:
     runs-on: ubuntu-latest
     permissions:

docker/compose.test.yml

@@ -0,0 +1,21 @@
+services:
+
+  postgres:
+    image: postgres:${PG_VERSION}
+    environment:
+      - PG_VERSION=latest
+      - POSTGRES_PASSWORD=secret
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+
+  pg_back:
+    build: ..
+    environment:
+      - PGPASSWORD=secret
+    command: -h postgres -p 5432 -U postgres
+    depends_on:
+      postgres:
+        condition: service_healthy