image-builder: retry docker build once on failure

The -current full build failed on 2026-09-23 after all ten steps had
passed, at layer export, losing a race on the containerd snapshotter's
content lock ("failed to open writer: ref ... locked for 74ms ...
unavailable"). Disk was not the issue this time (73G free). The build
cache pre-prune that works around this only runs under --force, so the
normal gated nightly build had no guard.

Add docker_build() to lib.sh: docker build, retried once. By the time
the export fails every step is cached, so the retry is little more than
a re-export. Used by build-full-image.sh and build-sbo-testbuild.sh,
the two scripts that export ~20-33G images; bootstrap.sh's image is
small and has never hit this.

Self-check covers success, one failure, and two failures with a stub.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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