Finally and thanks to @KenS I was able to have a solution to my problem.

I'm not able to get the latest master branch built for alpine linux. It would take too much time to find a solution to this - which I don't have.

So I took the latest release (10.04 for now), which I am able to build for alpine and patched in the following commit -> https://cgit.ghostscript.com/cgi-bin/cgit.cgi/ghostpdl.git/commit/?id=5d9b000c70f66f5be7bfaed1f16ef4e50d820dc6

The result is a fully valid PDF/A-3 document and also Adobe sees the attachment now.

So until 10.05 will be released in march this is fine for us.