Build Docker images in GitLab CI

Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk>

.gitlab-ci.yml

+stages:
+  - docker
+
+docker:
+  stage: docker
+  image:
+    name: gcr.io/kaniko-project/executor:debug
+    entrypoint: [""]
+  script:
+    - |
+      cat << EOF > /kaniko/.docker/config.json
+      {
+        "auths":{
+          "$CI_REGISTRY": {
+            "username":"$CI_REGISTRY_USER",
+            "password":"$CI_REGISTRY_PASSWORD"
+          }
+        }
+      }
+      EOF
+    - >
+      /kaniko/executor
+      --context $CI_PROJECT_DIR
+      --dockerfile $CI_PROJECT_DIR/Dockerfile
+      --destination $CI_REGISTRY_IMAGE/obs-api:$CI_COMMIT_REF_SLUG
+      --build-arg REGISTRY=$CI_REGISTRY_IMAGE
+      --single-snapshot
+    - echo Pushed