I'm trying to start a simple Flask "Hello world" app in a docker container but I keep getting this error: "OSError: [Errno 8] Exec format error: '/app/app.py'"

My host operating system is Windows 10.

My Dockerfile:

```
FROM python:3.6

ENV PYTHONBUFFERED 1

ADD . /app

WORKDIR /app

RUN pip install -r requirements.txt

```

I have requirements.txt with Flask==1.0.2.

app.py:

```
from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
    return "Hello World!"

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=8000, debug=True)

```

and docker-compose.yml:

```
version: '3'

services:
  app:
    build: .
    command: python app.py
    ports:
      - "8000:8000"

```

Whole log of container:

```
app_1  |  * Serving Flask app "app" (lazy loading)
app_1  |  * Environment: production
app_1  |    WARNING: Do not use the development server in a production environment.
app_1  |    Use a production WSGI server instead.
app_1  |  * Debug mode: on
app_1  |  * Running on http://0.0.0.0:8000/ (Press CTRL+C to quit)
app_1  |  * Restarting with stat
app_1  | Traceback (most recent call last):
app_1  |   File "app.py", line 9, in <module>
app_1  |     app.run(host='0.0.0.0', port=8000, debug=True)
app_1  |   File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 943, in run
app_1  |     run_simple(host, port, self, **options)
app_1  |   File "/usr/local/lib/python3.6/site-packages/werkzeug/serving.py", line 988, in run_simple
app_1  |     run_with_reloader(inner, extra_files, reloader_interval, reloader_type)
app_1  |   File "/usr/local/lib/python3.6/site-packages/werkzeug/_reloader.py", line 332, in run_with_reloader
app_1  |     sys.exit(reloader.restart_with_reloader())
app_1  |   File "/usr/local/lib/python3.6/site-packages/werkzeug/_reloader.py", line 176, in restart_with_reloader
app_1  |     exit_code = subprocess.call(args, env=new_environ, close_fds=False)
app_1  |   File "/usr/local/lib/python3.6/subprocess.py", line 287, in call
app_1  |     with Popen(*popenargs, **kwargs) as p:
app_1  |   File "/usr/local/lib/python3.6/subprocess.py", line 729, in __init__
app_1  |     restore_signals, start_new_session)
app_1  |   File "/usr/local/lib/python3.6/subprocess.py", line 1364, in _execute_child
app_1  |     raise child_exception_type(errno_num, err_msg, err_filename)
app_1  | OSError: [Errno 8] Exec format error: '/app/app.py'
flaskdockerproject_app_1 exited with code 1

```

UPDATE

After I added the shebang in app.py like @larsks said I'm getting this error:
"FileNotFoundError: [Errno 2] No such file or directory: '/app/app.py': '/app/app.py'.

All the files are in the container and in the right place.