install zlib1g-dev to make php zip extension installable

see docker-library/php#61

Author: alfredbez

Dockerfile

@@ -1,6 +1,6 @@
 FROM php:7.1.30
 
-RUN apt-get update && apt-get install -y ssh libffi-dev python-dev libssl-dev rsync python-pip python-setuptools && \
+RUN apt-get update && apt-get install -y zlib1g-dev ssh libffi-dev python-dev libssl-dev rsync python-pip python-setuptools && \
 	docker-php-ext-install bcmath && \
 	docker-php-ext-install zip && \
 	pip install -U pip setuptools && \