ci: split Docker base image builds

[ibodrov]

Summary

• add a weekly/manual docker-base-images workflow for concord-base and concord-ansible
• add Buildx bake groups for base-image and app-image builds
• change docker-multiarch release dispatch to build app images from published base images resolved to digests
• update release docs for the base_docker_tag input

Verification

• git diff --check
• ruby YAML parse for docker-base-images.yml and docker-multiarch.yml
• docker buildx bake --print for base-images and app-images groups