php extension install readme update

php_extension_install.md

@@ -1,12 +1,29 @@
 # PHP Extensions needed
 
 * php-mbstring
+* php-gd
 
 
 ## install php extensions in docker
 
+### alpine
+
 ```
 docker exec -it {container id} sh
 apk add php-mbstring
 ...
 ```
+
+### php-fpm:alpine
+
+```
+docker exec -it {container id} sh
+apk add \
+    zlib-dev \
+    libpng-dev \
+    freetype-dev \
+    libjpeg-turbo-dev \
+    && docker-php-ext-configure gd --with-freetype --with-jpeg \
+    && docker-php-ext-install -j$(nproc) gd
+...
+```