image-builder: feed full build from LAN NFS mirror over HTTP, fix GPG verify

The full-image build pulled the entire Slackware tree from slackware.nl over
WAN: slow (40+ min), flaky (truncated .asc downloads), and it wedged the
legacy builder. The identical tree is on LAN NFS, so serve it to the build
container instead.

- config: add HTTP_MIRROR_HOST/PORT (docker0 bridge gateway + ephemeral port).
- bootstrap.sh: bake the LAN HTTP mirror URL into the base image instead of
  slackware.nl, so the full build inherits it.
- build-full-image.sh: start an ephemeral python3 http.server rooted at the
  variant NFS tree on the bridge for the build duration, killed via the EXIT
  trap; require_mount so an unmounted NAS fails loud; wait for the server
  before building.

Also fix a silent-failure bug: without importing the Slackware GPG key against
the mirror first, slackpkg cannot verify CHECKSUMS.md5 and installs almost
nothing (94 packages instead of ~1788). Add "slackpkg update gpg" as the first
Dockerfile step; verified the package count jumps 94 -> 1788 with it.

Fetch is incremental (slackpkg downloads only installed packages), unlike the
buildkit --build-context path which eagerly copies the whole multi-GB tree.

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

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