Update apt sources before installing imagemagick

laradock#1512

Author: jeliasson

workspace/Dockerfile

@@ -654,6 +654,7 @@ USER root
 ARG INSTALL_IMAGEMAGICK=false
 
 RUN if [ ${INSTALL_IMAGEMAGICK} = true ]; then \
+    apt-get update && \
     apt-get install -y --force-yes imagemagick php-imagick \
 ;fi