Django server connection error with Docker Toolbox in OS X Yosemite - django

Question

I run the following yml file with $docker-compose up command. I'm developing a REST API with django (Udemy "Build a Backend REST API with Python & Django - Advanced").
System: OS X 10.10.5
Docker version: 18.03.0-ce
docker-compose version: 1.20.1
Q: I am unable to access localhost with 127.0.0.1
docker-compose.yml
version: "2.2"
services:
app:
build:
context: .
ports:
- "8000:8000"
volumes:
- ./app:/app
command: >
sh -c "python manage.py wait_for_db &&
python manage.py migrate &&
python manage.py runserver 0.0.0.0:8000"
environment:
- DB_HOST=db
- DB_NAME=app
- DB_USER=postgres
- DB_PASS=supersecretpassword
depends_on:
- db
db:
image: postgres:10-alpine
environment:
- POSTGRES_DB=app
- POSTGRES_USER=postgres
- POSTGRES_PASSWORD=supersecretpassword
Dockerfile
FROM python:3.7-alpine
#python unbuffered environmental variable
ENV PYTHONUNBUFFERED 1
#copy requirements.txt from the directory adjacent to
#the Dockerfile into dockerimage/requirments.txt
COPY ./requirements.txt /requirements.txt
RUN apk add --update --no-cache postgresql-client
RUN apk add --update --no-cache --virtual .tmp-build-deps \
gcc libc-dev linux-headers postgresql-dev
RUN pip install -r /requirements.txt
RUN apk del .tmp-build-deps
#create an empty folder named app in docker image
RUN mkdir /app
WORKDIR /app
COPY ./app /app
RUN adduser -D user
USER user
Some suggest to add CMD ["python", "manage.py", "runserver",
"0.0.0.0:8000"] in Dockerfile; didn't work in this case. In django
settings.py, ALLOWED_HOSTS = ['192.168.99.100']
Console output (part of: see bold)
File "/usr/local/lib/python3.7/site-packages/django/db/backends/postgresql/base.py",
line 178, in get_new_connection connection = Database.connect(conn_params)
File "/usr/local/lib/python3.7/site-packages/psycopg2/init.py",
line 130, in connect conn = _connect(dsn, connection_factory=connection_factory, kwasync)
django.db.utils.OperationalError: could not connect to server: Connection refused **
Is the server running on host "db" (172.19.0.2) and accepting
TCP/IP connections on port 5432?
I'm super new to this stuff, appreciate your help and suggestions.