Add php-mbstring

Dockerfile

@@ -3,7 +3,7 @@ MAINTAINER Girish Ramakrishnan <girish@cloudron.io>
 
 # this can be removed in next base image update
 RUN apt-get update && \
-    apt-get install -y php-zip crudini php-soap php-imap zip && \
+    apt-get install -y php-mbstring php-zip crudini php-soap php-imap zip && \
     rm -rf /var/cache/apt /var/lib/apt/lists
 
 RUN mkdir -p /app/code