Add Dockerfile for PHP Code Sniffer

phpcs/Dockerfile

@@ -0,0 +1,16 @@
+FROM debian:stretch-slim
+
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends \
+    	curl \
+	git \
+    	php \
+	zip \
+	unzip \
+	php-zip \
+	php-xml \
+	composer \
+    && rm -rf /var/lib/apt/lists/*
+
+RUN composer global require "squizlabs/php_codesniffer=*"
+CMD ["/root/.composer/vendor/bin/phpcs", "/data"]