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>

zsh/zshrc

@@ -2,19 +2,6 @@
 # overwritten on the next bootstrap run. Edit zsh/zshrc in the dotfiles repo
 # instead.
 
-# If this account was added to the docker group (dotfiles/docker/install.sh)
-# but this shell's session predates that, re-exec via newgrp so docker works
-# without a full logout. Guarded: fires only when the group is on the account
-# yet missing from the running shell's groups, so the newgrp'd shell (which
-# has the group) cannot loop.
-# Only for real terminals ([ -t 0 ]): newgrp replaces the shell and would
-# swallow the command in scripted `zsh -i -c ...` invocations.
-if [ -t 0 ] && [ -S /var/run/docker.sock ] \
-   && id -nG "${USER:-$(id -un)}" 2>/dev/null | grep -qw docker \
-   && ! id -Gn 2>/dev/null | grep -qw docker; then
-  exec newgrp docker
-fi
-
 # Userland tools installed by dotfiles/bin/install.sh live here.
 case ":$PATH:" in
   *":$HOME/.local/bin:"*) ;;