chore: 2542 reduce backend docker image size

[Sepehr-Sobhani]

ISSUE 2542

Reduces the backend image size from 2.5GB to 900MB

CHANGES:

• Uses multi-stage build
• Uses python--slim image (This required adding some packages manually)

References to achieve this:

• https://dylancastillo.co/til/django-poetry-dockerfile.html
• https://www.docker.com/blog/how-to-dockerize-django-app/
• https://betterstack.com/community/guides/scaling-python/dockerize-django/#step-3-writing-a-dockerfile-for-your-django-app
• https://testdriven.io/blog/docker-best-practices/#use-multi-stage-builds
• https://hadolint.github.io/hadolint/

[Sepehr-Sobhani]

@pbastia I made a few small tweaks to the env variables. Whenever you get a chance, could you please take a look? No rush at all! Thanks.