Converted Dockerfile to multistage Dockerfile.

[sourabhdeshmukh]

Stage 1 includes
Python 3.7 image.
Pythonpuffered=1.
Set argument version=master
Create app directory.
Set working directory to app.
Install all the python requirements.
$ python manage.py collectstatic builds assets and moves them to static directory.
Stage 2 includes
Get stage 1 image.
Set app as woking directory.
Expose port 8000 to access service.
Run the app.

• With pull requests:
  • Open your pull request against master or develop if exists.
  • Your pull request should have no more than two commits, if not you should squash them.
  • It should pass all tests in the available continuous integrations systems such as TravisCI.
  • You should add/modify tests to cover your proposed code changes.
  • If your pull request contains a new feature, please document it on the README.