Add boilerplate code, Docker environment, Makefile etc.

.drone.yml

@@ -0,0 +1,25 @@
+---
+kind: pipeline
+type: docker
+name: default
+
+steps:
+  - name: install dependencies
+    image: composer
+    volumes:
+      - name: composer-cache
+        path: /tmp/cache
+    environment:
+      COMPOSER_CACHE_DIR: /tmp/cache
+    commands:
+      - composer install --no-progress --no-interaction
+
+  - name: run unit tests
+    image: php:7.3
+    commands:
+      - vendor/bin/phpunit -c phpunit.xml
+
+volumes:
+  - name: composer-cache
+    host:
+      path: /tmp/drone/composer-cache