Fix Docker workflow for forked PSPDEV builds

[doodlewind]

Summary

• use the public upstream psp-packages image as the Docker build base in this fork
• grant the Docker workflow package write permission for GHCR pushes
• give the Dockerfile a valid default base image to avoid BuildKit's empty FROM warning

Notes

• the failed master Docker workflow was resolving ghcr.io/doodlewind/psp-packages:latest, which does not exist