Update README.md (mlocati#659)

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Author: Legion112

README.md

@@ -53,7 +53,7 @@ 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
-```
+```Dockerfile
 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
 ```