Install php-mbstring as it is required by PHPUnit

Dockerfile

@@ -40,6 +40,7 @@ RUN \
     php${PHP_VERSION}-cli \
     php${PHP_VERSION}-common \
     php${PHP_VERSION}-curl \
+    php${PHP_VERSION}-mbstring \
     php${PHP_VERSION}-mysql \
     php${PHP_VERSION}-sqlite3 \
     php${PHP_VERSION}-xml \