Add gd extension for pandoc in Dockerfile

[kittysnacks]

Before this change:

Interactive shell

php > $img = imagecreate(12,12);

Warning: Uncaught Error: Call to undefined function imagecreate() in php shell code:1
Stack trace:
#0 {main}
  thrown in php shell code on line 1
php >

After this change:

Interactive shell

php > $img = imagecreate(12,12);
php > var_dump($img);
object(GdImage)#1 (0) {
}
php >

✅ Tested in staging.