Switch to matrix syntax for pipeline

.gitlab-ci.yml

@@ -12,14 +12,14 @@ php:
       #  PHP: 7.0
       #- ALPINE: 3.7
       #  PHP: 7.1
-      - ALPINE: 3.8
-        PHP: 7.2
-      - ALPINE: 3.11
-        PHP: 7.3
-      - ALPINE: 3.11
-        PHP: 7.4
-      - ALPINE: 3.12
-        PHP: 8.0
+      - ALPINE: '3.8'
+        PHP: '7.2'
+      - ALPINE: '3.11'
+        PHP: '7.3'
+      - ALPINE: '3.11'
+        PHP: '7.4'
+      - ALPINE: '3.12'
+        PHP: '8.0'
   script:
     - docker build --pull --build-arg ALPINE_VERSION=${ALPINE} --build-arg PHP_MAJOR_VERSION=${PHP} --build-arg VERSION=${VERSION} -t $CI_REGISTRY_IMAGE:php-${PHP} .
     - docker push $CI_REGISTRY_IMAGE:php-${PHP}