Offer only what this machine can deliver

The installer knew one distribution: Debian, with docker, on a new enough
release. Everything else it would have offered and then failed at.

Three facts now decide what the matrix offers, and each is read from the
machine rather than assumed:

- The container runtime. Docker where the machine has one, podman on the Red
  Hat family, which ships no docker at all. Both go through the same compose
  plugin: compose speaks the Docker API and podman serves it, so there is one
  compose file and one deployment path, not two. Telling a Fedora operator to
  add Docker's own repository to a machine that already has a container
  runtime would have been the wrong trade.

- glibc. The server binary is downloaded, not built here, and it is linked
  against 2.39. Rocky 9 (2.34), Debian 12 and Ubuntu 22.04 cannot run it, so
  the host shape is refused there with the version found and the container
  shape named as the answer -- rather than installing a file that cannot
  start.

- The operating system itself. This compiles for macOS and Windows because Go
  compiles anything, and on either it would read no os-release, find no
  systemd, and describe a machine that does not exist. It now says what it is
  and exits.

Two bugs the other distributions found, both of which Debian could not have:

- The survey reported the first thing in the way and stopped, so on Fedora it
  asked to start podman.socket, and then -- having done it -- asked for the
  compose plugin. Needs are named now, not described, and reported together.

- apply used the survey taken before dependencies were installed, so on a
  machine that had no runtime at all it installed podman and then reached for
  docker. It re-surveys after resolving, and stops if containers still are
  not usable.

The lab takes DISTRO now: debian13, debian12, ubuntu2404, fedora, rocky9,
arch, each with its own disk and ssh port so several can be up at once. The
cases no longer say "docker" either. install-local passes on Debian 13,
Fedora 43 and Rocky 9 -- 20 checks each, ending with a sign-in to the webmail
the installer put there.

README.md

@@ -11,6 +11,13 @@ container or on the host, in any mixture.
 It only ever installs here. A second machine runs it too, and `inbuxa join`
 points that machine at a server already running elsewhere.
 
+Linux only, and it says so on any other system rather than reporting a
+machine that does not exist. Debian, Red Hat and Arch families, and the
+derivatives people run: Ubuntu, Fedora, Rocky, CentOS, CachyOS. It uses
+whichever container runtime the distribution ships -- docker where there is
+one, podman on the Red Hat family -- and refuses a shape the machine cannot
+deliver, with the reason.
+
 ## What works today
 
 This is early. What is built:
@@ -56,6 +63,7 @@ The installer writes units, creates users and takes ports 25 and 443, so it
 is tested on a throwaway virtual machine rather than on anybody's desk:
 
     e2e/vm/up.sh                              a Debian 13 machine, in qemu, as you
+    DISTRO=fedora e2e/vm/up.sh                or fedora, rocky9, ubuntu2404, arch, debian12
     e2e/vm/run.sh e2e/cases/survey.sh         what it says about a machine
     e2e/vm/run.sh e2e/cases/deps.sh           the offer, and taking it
     e2e/vm/run.sh e2e/cases/install-local.sh   a whole suite, and signing in to it