Add Sodium support for PHP version 7.2

Dockerfile

@@ -81,6 +81,9 @@ RUN echo 'hosts: files dns' > /etc/nsswitch.conf && \
     else \
       apk add --no-cache php7-fileinfo php7-tokenizer php7-simplexml php7-xmlreader php7-xmlwriter ; \
     fi && \
+    if [ "${PHP_VERSION}" = "7.2" ]; then \
+      apk add --no-cache php7-sodium ; \
+    fi && \
     rm /etc/apk/repositories && \
     mv /etc/apk/repositories.org /etc/apk/repositories && \
     apk update && \