Install Docker in experimental Void images

Make the local-only void-exp rootfs useful as a dev VM baseline by baking Docker and Compose into the XBPS package set instead of leaving container setup to manual follow-up.

Enable the docker runit service during image assembly, add a small boot preflight that loads the needed netfilter/overlay modules and applies the Docker sysctl file before dockerd starts, and keep the Void cleanup path removing caches, docs, and stale get-docker artifacts.

Refresh the README and repo guidance to describe Docker as part of the current Void image contract and to remind users that they need to rebuild and recreate Void VMs to pick it up.

Verified with bash -n make-rootfs-void.sh and git diff --check for the touched files. I did not run a live make rootfs-void or boot a fresh Void VM in this pass.

README.md

@@ -325,13 +325,14 @@ This path is intentionally local-only and does not change the default Debian
 image flow. It reuses the current runtime bundle kernel, initrd, and modules,
 but builds a lean `x86_64-glibc` Void userspace with:
 - `bash` installed for interactive/admin use
+- `docker` plus `docker-compose` installed from Void packages
+- the `docker` runit service enabled, with Docker netfilter/forwarding kernel prep
 - `openssh` enabled under runit
 - the bundled `banger-vsock-agent` health agent enabled under runit
 - `root` normalized to `/bin/bash` while keeping `/bin/sh` as the distro's system shell
 - a generated `/root` work-seed for fast creates
 
 It does not install the Debian-oriented extras from rebuilt default images:
-- no Docker
 - no `mise`
 - no `opencode`
 - no tmux plugin defaults
@@ -354,6 +355,10 @@ make void-register
 ./banger vm ssh void-dev
 ```
 
+Rebuild the Void rootfs and recreate existing `void-exp` VMs after changing the
+package set or guest provisioning; restart alone will not update the image
+contents or `/root` work-seed.
+
 There is also a smoke path for the experimental image:
 ```bash
 make verify-void