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>

.agents/skills/add-bootstrap-module/SKILL.md

@@ -44,9 +44,9 @@ Every module's `install.sh` satisfies all of these. Copy the shape from
 ## Wiring it up
 
 **`bootstrap.sh`** — add the call. Order is load-bearing and each existing
-out-of-order entry carries a comment explaining why (`docker/` before `bin/`
-so kind's gate sees the daemon; `docker-rootless-cgroup/` last because it
-needs the rootless daemon already up). If your module has an ordering
+out-of-order entry carries a comment explaining why (`secrets/` after `bin/`
+because it needs `op`; `docker-rootless-cgroup/` last because it needs the
+rootless daemon already up). If your module has an ordering
 constraint, add it *with* a comment; if it doesn't, append it near related
 modules.
 
@@ -61,7 +61,7 @@ Never commit the value itself.
 ## Non-systemd hosts
 
 Kasm has no systemd. Anything that would `systemctl enable` must degrade
-gracefully — see `docker/install.sh`, which detects the situation and no-ops
+gracefully — see `docker-rootless-cgroup/install.sh`, which detects the situation and no-ops
 rather than failing the whole bootstrap run.
 
 ## Before you're done