image-builder: catch build/push failures, prune cache on --force

The main loop runs build_variant in an `if ! (...)` condition, which
suppresses set -e inside the subshell, so a failed docker build fell through
to push a nonexistent tag and log "Done" (seen live: a transient containerd
export lock reported success). Add explicit exit checks on docker build and
docker push in all three scripts; return non-zero so the variant is marked
failed.

Also prune the build cache before a --force full-image build: --force already
implies --no-cache, so the stale cache is dead weight that still loads the
snapshotter's lock bookkeeping and has raced the layer export. Best-effort,
never fails the build.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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