php 5.6 skip install deciman extension

php-fpm/Dockerfile

@@ -1078,10 +1078,14 @@ USER root
 ARG INSTALL_PHPDECIMAL=false
 
 RUN if [ ${INSTALL_PHPDECIMAL} = true ]; then \
-    apt-get update -yqq \
-    && apt-get install -y libmpdec-dev \
-    && pecl install decimal \
-    && docker-php-ext-enable decimal \
+    if [ $(php -r "echo PHP_MAJOR_VERSION;") = "5" ]; then \
+      echo 'decimal not support PHP 5.6'; \
+    else \
+      apt-get install -yqq libmpdec-dev \
+      && pecl install decimal \
+      && docker-php-ext-enable decimal \
+      && php -m | grep -q 'decimal' \
+    ;fi \
 ;fi
 
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