GAE ERROR :- /bin/sh: 1: exec: gunicorn: not found

for me the error was as simple as making sure gunicorn was in requirements.txt

Flask==1.0.2
gunicorn==19.9.0

Note:

I see the OP had added this flag; this is to help others that may be running into exec: gunicorn: not found

A few changes and I was able to run your app in docker.

1. In Twilio_Routing.py , change host to listen on 0.0.0.0 instead of 127.0.0.1.This is needed to to have the server available externally as well.

2. Since your app.yaml is already configured, you don't need to customize your Dockerfile as Google App Engine requires. Keep it as your own custom one.Here's what I used:
   

   #Python's Alpine Base Image
   FROM python:3.6-alpine3.6
   
   #Installing all python modules specified
   ADD requirements.txt requirements.txt
   RUN pip install -r requirements.txt
   
   #Copy App Contents
   ADD . /app
   WORKDIR /app
   
   #Start Flask Server
   CMD [ "python","Twilio_Routing.py"]
   #Expose server port
   EXPOSE 8080