image-builder: abort early when the docker daemon is unreachable

A dead dockerd is a global failure, not a per-variant one, but every stage
treated it as the latter: the digest probes read an unreachable daemon as
"no digest, rebuilding to be safe", the build then failed, and the variant
loop logged "WARNING: variant X failed; continuing" and moved on. Cron kept
exiting nonzero into a log nobody read, so a daemon that died in August went
unnoticed for 25 days while no image was ever rebuilt.

Add require_docker() to lib.sh, following the existing require_mount()
precedent, and call it before the variant loop in all three stages. One loud
error, exit 1, instead of a nightly pile of warnings.

In build-full-image.sh the check goes before the --force cache prune, since
that prune also talks to the daemon.

Self-check stubs `docker` so it never contacts a real daemon.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AD4jP4wFMpBgEvd9ZP9Xhu

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