gitlab-ci: Test with Docker Compose and DinD

Use the Docker-in-Docker service to bring up a whole OBS instance using
Docker Compose using the produced images and test connectivity.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

.gitlab-ci.yml

 stages:
   - prepare
   - docker
+  - test
 
 prepare:
   stage: prepare
@@ -59,3 +60,39 @@ build-docker-worker:
   extends: .build-docker-image
   variables:
     image: worker
+
+test:
+  image: debian:bullseye-slim
+  services: [ 'docker:stable-dind' ]
+  variables:
+    # make things a bit faster
+    DOCKER_DRIVER: overlay2
+    # disable TLS in the daemon
+    DOCKER_TLS_CERTDIR: ''
+    # point the client to the plaintext tcp port
+    DOCKER_HOST: tcp://docker:2375
+  before_script:
+    - apt update && apt install -y --no-install-recommends
+        docker-compose
+        docker.io
+        gnupg2
+        osc
+        pass
+        wait-for-it
+    - 'sed -i -e "s|image: .*/docker-obs/\(.*\):latest|image: $CI_REGISTRY_IMAGE/\1:$TAG|" docker-compose.prod.yml'
+    - cat docker-compose.prod.yml
+    - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" "$CI_REGISTRY"
+    - docker-compose -f docker-compose.yml -f docker-compose.prod.yml up -d
+  script:
+    - mkdir -p ~/.config/osc/
+    - |
+      cat > ~/.config/osc/oscrc <<EOF
+      [general]
+      apiurl = https://docker
+      [https://docker]
+      sslcertck = 0
+      user = Admin
+      pass = opensuse
+      EOF
+    - wait-for-it docker:443 --timeout=120 --strict -- docker-compose -f docker-compose.yml -f docker-compose.prod.yml logs
+    - osc who