lock pacakge version to fix dep breakage

.drone.yml

@@ -5,7 +5,7 @@ name: testsuite
 steps:
 
 - name: composer install
-  image: chialab/php
+  image: chialab/php:7.3
   commands:
   - composer install
   volumes:
@@ -13,21 +13,21 @@ steps:
   path: /root/.composer/cache/
 
 - name: wait for test server
-  image: alpine
+  image: alpine:3.12
   commands:
   - echo "Waiting for server to launch on testserver:8080..."
   - while ! nc -z testserver 8080; do sleep 0.1 ; done
   - echo "Ready!"
 
 - name: run tests
-  image: chialab/php
+  image: chialab/php:7.3
   commands:
   - URL=http://testserver:8080 vendor/bin/phpunit tests --testdox --color=always --no-interaction
 
 services:
 
 - name: testserver
-  image: node
+  image: node:15.2
   detach: true
   commands:
   - cd tests/server/