Update README.md (mlocati#658)

Author: Legion112

README.md

@@ -52,6 +52,11 @@ COPY --from=mlocati/php-extension-installer /usr/bin/install-php-extensions /usr
 
 RUN install-php-extensions gd xdebug
 ```
+Alternative that does not increase the image size with the tool
+```
+RUN  --mount=type=bind,from=mlocati/php-extension-installer:1.5,source=/usr/bin/install-php-extensions,target=/usr/local/bin/install-php-extensions \
+      install-php-extensions pcntl
+```
 
 > **Warning**: by using this method you may use an outdated version of the `mlocati/php-extension-installer` image.
 >