give full path to echo

https://github.com/docker/docker/issues/8949

Dockerfile

@@ -13,7 +13,7 @@ RUN mkdir -p /run/wordpress/sessions && \
 
 RUN curl -L -o /app/code/wp https://github.com/wp-cli/wp-cli/releases/download/v0.24.1/wp-cli-0.24.1.phar
 RUN chmod +x /app/code/wp
-RUN echo -e "apache_modules:\n  - mod_rewrite" > /app/code/wp-cli.yml
+RUN /bin/echo -e "apache_modules:\n  - mod_rewrite" > /app/code/wp-cli.yml
 
 
 # Get the plugins