v3.28.0

Changed:

• docker spec, #245:
  • explain various core base stages
  • explain GHCR vs Dockerhub
  • use Opencontainers labels
  • improve image naming rules
  • match ocrd-tool.json dockerhub name
  • improve example
• ocrd_eval spec: Clarify reading order evaluation, #247

Added:

• ci spec on best practices for Continuous Integration, #248
• code_review spec on best practices for code review, #248