Add build image and launch from source in README (#2658)

### What problem does this PR solve?

Move the build image and launch from source back to README.

### Type of change

- [x] Documentation Update

---------

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

docker/launch_backend_service.sh

@@ -0,0 +1,28 @@
+#!/bin/bash
+
+# unset http proxy which maybe set by docker daemon
+export http_proxy=""; export https_proxy=""; export no_proxy=""; export HTTP_PROXY=""; export HTTPS_PROXY=""; export NO_PROXY=""
+
+export LD_LIBRARY_PATH=/usr/lib/x86_64-linux-gnu/
+
+PY=python3
+if [[ -z "$WS" || $WS -lt 1 ]]; then
+  WS=1
+fi
+
+function task_exe(){
+    while [ 1 -eq 1 ];do
+      $PY rag/svr/task_executor.py $1;
+    done
+}
+
+for ((i=0;i<WS;i++))
+do
+  task_exe  $i &
+done
+
+while [ 1 -eq 1 ];do
+    $PY api/ragflow_server.py
+done
+
+wait;