fix: use Docker convenience script for Ubuntu/Debian

The manual apt repo setup failed because uname -m returns x86_64 but
Docker repo expects amd64 as arch name. Use get.docker.com which
handles arch/codename/GPG correctly. Also added proper arch mapping
(x86_64→amd64, aarch64→arm64) for future use.

install.sh

@@ -36,7 +36,12 @@ detect_os() {
 }
 
 OS=$(detect_os)
-ARCH=$(uname -m)
+RAW_ARCH=$(uname -m)
+case "$RAW_ARCH" in
+  x86_64)  ARCH="amd64"  ;;
+  aarch64) ARCH="arm64"  ;;
+  *)       ARCH="$RAW_ARCH" ;;
+esac
 
 # ── Git ──────────────────────────────────────────
 if ! command -v git &>/dev/null; then
@@ -66,15 +71,7 @@ if $NEED_DOCKER; then
   info "Installing Docker Engine..."
   case "$OS" in
     ubuntu|debian)
-      sudo apt update -qq
-      sudo apt install -y -qq ca-certificates curl
-      sudo install -m 0755 -d /etc/apt/keyrings
-      sudo curl -fsSL https://download.docker.com/linux/"$OS"/gpg -o /etc/apt/keyrings/docker.asc
-      sudo chmod a+r /etc/apt/keyrings/docker.asc
-      echo "deb [arch=$ARCH signed-by=/etc/apt/keyrings/docker.asc] https://download.docker.com/linux/$OS $(. /etc/os-release && echo "$VERSION_CODENAME") stable" \
-        | sudo tee /etc/apt/sources.list.d/docker.list > /dev/null
-      sudo apt update -qq
-      sudo apt install -y -qq docker-ce docker-ce-cli containerd.io
+      curl -fsSL https://get.docker.com | sh
       ;;
     arch|endeavouros)
       sudo pacman -S --noconfirm docker
@@ -97,8 +94,8 @@ if $NEED_DOCKER; then
   log "Docker Engine installed"
 fi
 
-if $NEED_COMPOSE && ! command -v docker &>/dev/null; then
-  # fresh install — compose plugin is already included in docker-ce package
+# Re-check compose — fresh Docker install may have included it
+if $NEED_COMPOSE && docker compose version &>/dev/null; then
   NEED_COMPOSE=false
 fi