drone pipeline: install dev dependencies for unit tests

.drone.yml

@@ -4,7 +4,22 @@ type: docker
 name: default
 
 steps:
-  - name: install
+  - 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:7.4
+    commands:
+      - vendor/bin/phpunit -c phpunit.xml
+
+  - name: remove dev dependencies
     image: composer
     volumes:
       - name: composer-cache
@@ -14,11 +29,6 @@ steps:
     commands:
       - composer install --no-progress --no-interaction --no-dev --optimize-autoloader
 
-  - name: unit tests
-    image: php:7.4
-    commands:
-      - vendor/bin/phpunit -c phpunit.xml
-
   - name: build image
     image: docker
     volumes: