Try a different approach

.gitlab-ci.yml

@@ -12,7 +12,8 @@ services:
 test:7:
     before_script:
       - sh build/docker_install.sh > /dev/null
-      - apk add --no-cache php7-phpdbg
+      - apk add --no-cache php7-apcu php7-memcached php7-phpdbg
+      - echo '\nextension=apcu.so\nextension=memcached.so' >> /etc/php7/php.ini
       - curl -sS https://getcomposer.org/installer | php
       - php composer.phar install --ignore-platform-reqs
     image: php:7-alpine

build/docker_install.sh

@@ -8,17 +8,5 @@ set -xe
 # Install git (the php image doesn't have it) which is required by composer
 echo -e 'http://dl-cdn.alpinelinux.org/alpine/edge/main\nhttp://dl-cdn.alpinelinux.org/alpine/edge/community\nhttp://dl-cdn.alpinelinux.org/alpine/edge/testing' > /etc/apk/repositories
 apk upgrade --update --no-cache \
-	g++ \
-	make \
-	automake \
 	curl \
-	git \
-	libmemcached-dev
-	
-echo -e 'apcu\nmemcached' >> /usr/src/php-available-exts
-	
-docker-php-source extract
-pecl install apcu-5.1.8 
-pecl install memcached-3.0.3
-docker-php-ext-enable apcu memcached
-docker-php-source delete
+	git