Fix Typo for RUNTIME_DISTRUBTION

Fix for GoogleCloudPlatform#529

Author: frost-byte

Diff for: php-base/Dockerfile

@@ -81,7 +81,7 @@ ENV NGINX_DIR=/etc/nginx \
 
 ARG RUNTIME_DISTRIBUTION="gcp-php-runtime-xenial-unstable"
 
-COPY ${RUNTIME_DISTRUBTION} /${RUNTIME_DISTRUBTION}
+COPY ${RUNTIME_DISTRIBUTION} /${RUNTIME_DISTRIBUTION}
 
 RUN mkdir -p $PHP_CONFIG_TEMPLATE
 COPY php-fpm.conf php.ini php-cli.ini "${PHP_CONFIG_TEMPLATE}/"