fix: multiarch Github action to have both x86 and arm64 images

[supermaxiste]

Our good old @rmfranken spotted a bug in the validation GH action causing the following error:

Unable to find image 'ghcr.io/sdsc-ord/shacl:latest' locally
latest: Pulling from sdsc-ord/shacl
docker: no matching manifest for linux/amd64 in the manifest list entries.

It looks like our current setup does not provide images for both arm and x86, instead the GitHub action overwrites the x86 image with the arm one, which is the only one available. After a bit of search I found out the correct approach to build multiarch images when we have different arguments involved: matrix!
This PR implements the matrix approach to allow proper multiarch images for shacl.

[cmdoret]

Awesome! This is much cleaner 👍

[rmfranken]

Hahah, you give me too much credit. Spotting this bug is like saying: The car isn't working, and getting the credit for the diagnosis that it was the leaky fuel pump. Thank you for finding the leaky fuel pump @supermaxiste .