install the rootless docker prerequisites with docker itself

Reported from a member's daemon failing: "rootless Docker needs these packages on
the host: uidmap".

They were being installed — but only inside branch [2] "rootless Docker for
<owner>" in section 22. The owner's choice is not the only one that matters: every
Developer account the platform provisions gets its own rootless daemon whatever the
owner picked for themselves. So on a machine where the owner chose the docker
group, the host never got them and every member's daemon failed.

Moved into install_docker_engine, so they arrive with Docker rather than with one
particular answer to a question about the owner.

Three packages, not the one in the error. checkDockerPrerequisites in
os-user-docker.ts is the authority and wants uidmap (newuidmap, newgidmap) AND
docker-ce-rootless-extras (dockerd-rootless-setuptool.sh); dbus-user-session is
what keeps a member's systemd --user alive without a login session. rootless-extras
is only RECOMMENDED by docker-ce — installed by default, so usually there by luck,
and absent on any host configured with --no-install-recommends. Named explicitly.

Reproduced on this machine while checking: rootless-extras present via Recommends,
uidmap absent, newuidmap and newgidmap missing. Exactly the reported failure, on a
box that chose the docker group.

The rootless branch still installs uidmap and dbus-user-session behind its
pkg_is_installed guard. Redundant now, kept deliberately: it is the only thing that
fixes a machine whose Docker was installed by an older run of this script.

Verified: bash -n on both files, and all three packages present in the noble
archive.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

scripts/setup/machine-setup/machine-setup.sh

@@ -1761,7 +1761,9 @@ if ! skip; then
   elif ! docker_is_installed; then
     echo ""
     echo "     to install:   docker-ce, the CLI, containerd, buildx and compose,"
-    echo "                   from Docker's own repository"
+    echo "                   from Docker's own repository — plus uidmap,"
+    echo "                   dbus-user-session and docker-ce-rootless-extras,"
+    echo "                   which every member's own rootless daemon needs"
     if confirm "Install it?"; then
       if install_docker_engine; then
         ok "$(docker --version 2>/dev/null | cut -d, -f1) installed"