CI: Require dind tag on Docker image build jobs

Add the 'dind' tag to .build:docker:linux:amd64, .build:docker:linux:arm64, and .build:docker:linux:riscv64.

This ensures Docker image builds only run on privileged runners that support Docker-in-Docker (our dedicated GCP Spot builders), preventing unprivileged community runners and standard GitLab SaaS runners from claiming and failing these jobs.
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