Auto-install Docker, add USB drive docs, improve whitelist UI

- lib/common.sh: require_docker now installs Docker CE + Compose plugin
  via get.docker.com instead of erroring out if Docker is missing.
  Also adds the calling user to the docker group automatically.

- README.md: fix 'tells you how to install Docker' → 'installs Docker
  automatically'; add full USB drive usage section (mount, fstab,
  DOCKER_DIR config, moving existing data, tips).

- services/minecraft.sh: replace single-source whitelist import with the
  multi-source UI from the updated setup-minecraft.sh — collects players
  from the current instance, saved backup files, and other servers' backups;
  assigns letters to each source so you can import by letter (all from that
  source) or by number (specific player).

https://claude.ai/code/session_017WJtGcE5jjerAQCUBWUE3H

lib/common.sh

@@ -153,8 +153,30 @@ require_root() {
 }
 
 require_docker() {
+    if command -v docker &>/dev/null; then
+        return 0
+    fi
+
+    log_info "Docker is not installed — installing now..."
+    if [ "$DRY_RUN" = true ]; then
+        echo "[DRY-RUN] Would install Docker CE and Docker Compose plugin via get.docker.com"
+        return 0
+    fi
+
+    # Official Docker convenience script — installs Docker CE + Compose plugin
+    if curl -fsSL https://get.docker.com | sh; then
+        if [ -n "$ACTUAL_USER" ] && [ "$ACTUAL_USER" != "root" ]; then
+            usermod -aG docker "$ACTUAL_USER" \
+                && log_info "Added $ACTUAL_USER to the docker group (re-login or run 'newgrp docker' to activate)"
+        fi
+        log_success "Docker installed ($(docker --version 2>/dev/null))"
+    else
+        log_error "Docker installation failed. Try manually: curl -fsSL https://get.docker.com | sh"
+        return 1
+    fi
+
     if ! command -v docker &>/dev/null; then
-        log_error "Docker is not installed. Install Docker first (run: $0 docker)."
+        log_error "Docker binary not found after install — something went wrong."
         return 1
     fi
 }