Remove unused Docker/CD workflow

[risu729]

Summary

• remove .github/workflows/docker.yml
• drop the unused Docker image build and CD dispatch automation from this repository

Why

• the file is a single combined workflow containing both the Docker build job and the deploy dispatch job
• the current deployment manifest for spooderman still points at ghcr.io/devsoc-unsw/spooderman:dummy, so this automation is not the active deployment path
• the workflow is not in a working state and keeping it around makes the repository look deployable when it is not

Verification

• confirmed .github/workflows/docker.yml was the only workflow handling Docker/CD
• confirmed the remaining workflows are rust-ci.yml and test-scraper.yml
• confirmed the deployment manifest at deployment/projects/spooderman/cronjob.yml still references a dummy image tag