I trying to deploy my app on GAE using their trial version. I was so far successful in creating an app.yaml with a custom settings for flexible environment with python 3.6.

However, while deploying the app, the app builds successfully, however, I keep getting the following error

Updating service [default] (this may take several minutes)...failed. ERROR: (gcloud.app.deploy) Error Response: [9] Application startup error: /bin/sh: 1: exec: gunicorn: not found

Following is the folder hierarchy of files in my project:

Following the code for app.yaml

env: flex
runtime: custom
api_version: 1
entrypoint: gunicorn -b :$PORT main:app
runtime_config:
    python_version: 3.6

#handlers:
#- url: /SmsResponse
#  script: Twilio_Routing.RecivedSms
#
#- url: /CallResponse
#  script: Twilio_Routing.ReceivedCall

I am surely missing out on something and I would really appreciate some help here. Link to git repo

requirements.txt

Flask==0.10.1
gunicorn==19.3.0
twilio==6.8.4

DockerFile

FROM gcr.io/google-appengine/python
LABEL python_version=python3.6
RUN virtualenv --no-download /env -p python3.6

# Set virtualenv environment variables. This is equivalent to running
# source /env/bin/activate
ENV VIRTUAL_ENV /env
ENV PATH /env/bin:$PATH

# Copy the application's requirements.txt and run pip to install all
# dependencies into the virtualenv.
ADD requirements.txt requirements.txt
RUN pip install -r requirements.txt

ADD . /app/

#CMD gunicorn -b :$PORT main:app
ENTRYPOINT [ "python", "Twilio_Routing.py" ]

P.S. After the changes for the requirements.txt, I am getting error 502 Bad Gateway.

Logs showing that the service was executed successfully.

017-12-25 01:29:03 default[20171224t212610]   * Running on http://127.0.0.1:8080/ (Press CTRL+C to quit)
2017-12-25 01:29:03 default[20171224t212610]   * Restarting with stat
2017-12-25 01:29:03 default[20171224t212610]   * Debugger is active!
2017-12-25 01:29:03 default[20171224t212610]   * Debugger PIN: 134-103-452
2017-12-25 01:29:17 default[20171224t212610]   * Running on http://127.0.0.1:8080/ (Press CTRL+C to quit)
2017-12-25 01:29:17 default[20171224t212610]   * Restarting with stat
2017-12-25 01:29:17 default[20171224t212610]   * Debugger is active!
2017-12-25 01:29:17 default[20171224t212610]   * Debugger PIN: 134-103-452

Can someone look at my code in git and tell me what is it that I am missing here?

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