Remove the docker module: dotfiles no longer installs Docker

Installing Docker Engine is a host/image decision, not a per-user
environment one. In Coder workspaces (unprivileged pods, no systemd) the
module ran get.docker.com on every start for a daemon that can never
run there. Where Docker is wanted, install it on the host or bake it
into the image.

Also drops the zshrc newgrp-docker hook that existed only for this
module's group change, and the docs/skills references to docker/.
docker-rootless-cgroup/ (configures Kasm's existing rootless daemon)
and bin/'s kind gate (detects a daemon) stay.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -32,14 +32,6 @@ Already have a clone? Just run it directly:
 - `git/` — sets git `user.name`/`user.email` and configures
   `credential.helper store` (plaintext `~/.git-credentials`, 0600) for
   HTTPS auth.
-- `docker/` — installs full (rootful) Docker Engine from Docker's upstream
-  package repo (get.docker.com on Debian-family; the docker-ce repo wired up
-  directly on RHEL-family, since the convenience script rejects EL rebuilds
-  like Oracle Linux by name), enables the service (systemd hosts), and adds
-  the invoking user to the
-  `docker` group (takes effect at next login). No-ops where a rootless
-  Docker daemon is already running (e.g. Kasm) so it never fights that
-  setup. Uses sudo.
 - `bin/` — installs userland devops CLIs as self-contained, statically
   linked release binaries into `~/.local/bin`: jq, kubectl, kubelogin (the
   `kubectl oidc-login` plugin, for OIDC auth to Omni clusters), helm, k9s,
@@ -50,6 +42,8 @@ Already have a clone? Just run it directly:
   coder CLI for Coder workspaces.
   kind is installed only when full (non-rootless) Docker is available —
   it doesn't work against rootless Docker, so it is skipped on Kasm.
+  Dotfiles does not install Docker itself; install it on the host (or bake
+  it into the image) where you want it.
   No sudo, nothing outside `$HOME`, so tools persist across Kasm
   redeploys with no reinstall. Idempotent: each tool records its version
   under `~/.local/bin/.versions` and is skipped when the pinned version