Enhancement: add SLSA provenance to the GHCR Container images

Restic's GitHub Container Registry (GHCR) image build workflow now includes
SLSA provenance generation. This enhancement improves the security and 
traceability of images built and pushed to GHCR. 

https://github.com/restic/restic/issues/4983
https://github.com/restic/restic/pull/4999