Add decimal extension

[gisostallenberg]

Expected Behavior

I would like to use the decimal extension in this images.
It requires libmpdec-dev to work.

Your Environment

• Version used: php8.1-v4

If you're willing to add this extension, I'm willing to provide a PR when I get some info on how to add an additional extension and it's dependencies.

[gisostallenberg]

@mistraloz Can you point me to a way to achieve this?