Fix iconv() warnings only for PHP 7.3, not 7.2

Dockerfile

@@ -85,8 +85,7 @@ RUN echo 'hosts: files dns' > /etc/nsswitch.conf && \
     apk add --no-cache \
         php7 php7-phar php7-json php7-dom php7-gd php7-mbstring php7-openssl \
         php7-pdo php7-curl php7-xml php7-zip php7-session php7-ctype php7-iconv \
-        php7-bcmath \
-        gnu-libiconv && \
+        php7-bcmath && \
     if [ "${PHP_VERSION}" = "7.0" ]; then \
       echo "no extra packages required" ; \
     else \
@@ -96,7 +95,7 @@ RUN echo 'hosts: files dns' > /etc/nsswitch.conf && \
       apk add --no-cache php7-sodium ; \
     fi && \
     if [ "${PHP_VERSION}" = "7.3" ]; then \
-      apk add --no-cache php7-sodium ; \
+      apk add --no-cache php7-sodium gnu-libiconv ; \
     fi && \
     rm /etc/apk/repositories && \
     mv /etc/apk/repositories.org /etc/apk/repositories && \