Add prerequisites script and update README (#24567)

Why I did it
Add prerequisites script for automated prerequisites installation and repository cloning. Update README with single script solution.

Author: qiluo-msft

README.md

@@ -82,7 +82,22 @@ Any server can be a build image server as long as it has:
 
 A good choice of OS for building SONiC is currently Ubuntu 22.04.
 
-## Prerequisites
+## Automated prerequisites installation and repository cloning
+
+For convenience, you can use the automated prerequisites script to handle both prerequisites installation and repository cloning:
+
+```shell
+curl -sSL https://raw.githubusercontent.com/sonic-net/sonic-buildimage/master/scripts/prerequisites.sh | bash
+```
+
+This script will automatically:
+* Install required packages (pip, jinja, Docker)
+* Configure Docker for non-root usage
+* Clone the repository with all submodules
+
+After completing this step, proceed to the [Usage](#usage) section below.
+
+## Manual prerequisites installation
 
 * Install pip and jinja in host build machine, execute below commands
    if j2/jinjanator is not available:
@@ -92,6 +107,9 @@ sudo apt install -y python3-pip
 pip3 install --user jinjanator
 ```
 
+> **Note:** If you cannot run the `j2` command after installation, this is likely because the `~/.local/bin` directory was just created and is not yet included in your `$PATH`. Please log out and log back in to refresh your environment, then test the command again.
+
+
 * Install [Docker](https://docs.docker.com/engine/install/) and configure your
   system to allow running the 'docker' command without 'sudo':
   * Add current user to the docker group: `sudo gpasswd -a ${USER} docker`
@@ -103,7 +121,7 @@ pip3 install --user jinjanator
 > This will avoid [known bugs that falsely report read-only filesystems issues](https://stackoverflow.com/questions/52526219/docker-mkdir-read-only-file-system)
 > during the build process.
 
-## Clone the repository with all the git submodules
+## Manual clone the repository with all the git submodules
 
 To clone the code repository recursively:
 

scripts/prerequisites.sh

@@ -0,0 +1,111 @@
+#!/usr/bin/env bash
+# Strict mode: exit on error, undefined variable, or pipeline failure
+set -euo pipefail
+
+###############################################
+# Error Handling
+###############################################
+trap 'echo "[ERROR] Script failed at line $LINENO. Exiting."; exit 1' ERR
+
+# Wrapper to provide better log messages for each step
+run_step() {
+    local msg="$1"
+    shift
+    echo "==> $msg"
+    "$@"
+    echo "==> Completed: $msg"
+}
+
+###############################################
+# Configurable Variables
+# Can be overridden via environment variables:
+#   SONIC_REPO=<url> SONIC_DIR=<path> BRANCH=<name> bash prerequisites.sh
+###############################################
+SONIC_REPO="${SONIC_REPO:-https://github.com/sonic-net/sonic-buildimage.git}"
+SONIC_DIR="${SONIC_DIR:-$HOME/sonic-buildimage}"
+BRANCH="${BRANCH:-master}"
+
+###############################################
+# Steps Begin
+###############################################
+
+run_step "Updating apt package index" \
+    sudo apt update
+
+run_step "Installing prerequisites (python3-pip, git)" \
+    sudo apt install -y python3-pip git
+
+run_step "Installing jinjanator (j2)" \
+    bash -c 'pip3 install --user jinjanator || sudo apt install j2cli'
+
+echo "==> Testing j2 availability..."
+if ! command -v j2 >/dev/null 2>&1; then
+    echo "[ERROR] j2 is not runnable."
+    echo "Please logout and login, then run the script again."
+    exit 1
+else
+    echo "==> j2 is available: $(command -v j2)"
+fi
+
+###############################################
+# Docker Install Check
+###############################################
+if ! command -v docker >/dev/null 2>&1; then
+    run_step "Installing Docker dependencies" \
+        sudo apt install -y ca-certificates curl gnupg lsb-release
+
+    run_step "Adding Docker GPG key" \
+        bash -c "curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo gpg --dearmor -o /usr/share/keyrings/docker-archive-keyring.gpg"
+
+    run_step "Adding Docker apt repository" \
+        bash -c "echo \
+        \"deb [arch=$(dpkg --print-architecture) signed-by=/usr/share/keyrings/docker-archive-keyring.gpg] \
+        https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable\" \
+        | sudo tee /etc/apt/sources.list.d/docker.list >/dev/null"
+
+    run_step "Updating apt after adding Docker repo" \
+        sudo apt update
+
+    run_step "Installing Docker Engine" \
+        sudo apt install -y docker-ce docker-ce-cli containerd.io
+else
+    echo "==> Docker already installed, skipping."
+fi
+
+run_step "Adding user '${USER}' to docker group" \
+    sudo gpasswd -a "${USER}" docker
+
+echo "==> Testing Docker availability without sudo..."
+if ! docker ps >/dev/null 2>&1; then
+    echo "[WARNING] Docker cannot be run without sudo yet."
+    echo "Please log out and log back in for the docker group membership to take effect."
+    echo "After logging back in, you can continue with the build process."
+else
+    echo "==> Docker is available without sudo."
+fi
+
+###############################################
+# Clone SONiC Repo
+###############################################
+if [ -d "$SONIC_DIR" ]; then
+    echo "==> Directory $SONIC_DIR already exists, skipping clone."
+else
+    run_step "Cloning SONiC repository" \
+        git clone --recurse-submodules "$SONIC_REPO" "$SONIC_DIR"
+fi
+
+cd "$SONIC_DIR"
+
+run_step "Checking out branch ${BRANCH}" \
+    git checkout "${BRANCH}"
+
+run_step "Fetching latest code" \
+    git fetch
+
+###############################################
+# Done
+###############################################
+echo "===================================================="
+echo "Done!"
+echo "===================================================="
+