ci: Add automated CLI documentation sync workflow

This adds a GitHub Actions workflow and supporting script to
automatically sync CLI documentation from the docker/cli repository on a
daily schedule.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

hack/sync-cli-docs.sh

@@ -0,0 +1,38 @@
+#!/bin/bash
+set -euo pipefail
+
+main() {
+    local branch_name="${1:-upstream/master}"
+    local cli_source="${2:-$HOME/src/cli}/.git"
+    local worktree_dir="./internal-update-cli-docs"
+
+    (
+        set -e
+        GIT_DIR="$cli_source"
+        GIT_DIR="$GIT_DIR" git fetch upstream
+        GIT_DIR="$GIT_DIR" git worktree add "$worktree_dir" "$branch_name"
+    ) || return $?
+    trap "GIT_DIR=\"$cli_source\" git worktree remove \"$worktree_dir\" --force 2>/dev/null || true" EXIT
+
+    (set -e; cd "$worktree_dir"; make -f docker.Makefile yamldocs || { printf "::error::Failed to generate YAML docs!\n"; exit 1; }) || return $?
+    cp "$worktree_dir"/docs/yaml/*.yaml ./data/engine-cli/
+
+    if git diff --quiet "./data/engine-cli/*.yaml"; then
+        printf "\e[32m✅ Already up to date\e[0m\n"
+        return 100
+    fi
+
+    echo -e "ℹ️ Changes detected:"
+    git diff --stat "./data/engine-cli/*.yaml" || true
+
+    NICE_GIT_REF=$(cd "$worktree_dir" && git describe --always --dirty) || return $?
+
+    git add "./data/engine-cli/*.yaml"
+
+    git commit -m "cli: sync docs with docker/cli $NICE_GIT_REF"
+    
+    printf "\e[32m✅ Committed changes\e[0m\n"
+    return 0
+}
+
+main "$@"