Fix Phalcon (#6363)

frameworks/PHP/phalcon/phalcon.dockerfile

@@ -5,7 +5,8 @@ ARG DEBIAN_FRONTEND=noninteractive
 RUN apt-get update -yqq && apt-get install -yqq software-properties-common > /dev/null
 RUN LC_ALL=C.UTF-8 add-apt-repository ppa:ondrej/php
 RUN apt-get update -yqq > /dev/null && \
-    apt-get install -yqq nginx git unzip php7.4 php7.4-common php7.4-cli php7.4-fpm php7.4-mysql php7.4-mongodb  > /dev/null
+    apt-get install -yqq nginx git unzip \
+    php7.4-cli php7.4-fpm php7.4-mysql php7.4-mongodb php7.4-mbstring > /dev/null
 
 RUN apt-get install -yqq composer > /dev/null