Auto Generation of django secret key

[IdirLISN]

Description:

Generation of a random secret key for django instance when building the app.
Write the value on .env.
each docker compose up -d, entrypoint.sh will read the variable in .env, if it doesn't exist it will generate one random secret key.
If a user want to deploy his own codabench instance, it will be secured with a unique secret key.
Enables django to use a private secret key for each new instance of it.

Issues this PR resolves

Secret key was clear and was the same for all instances. It means all codabench instances can authenticate to each other.

A checklist for hand testing

• just deploy and start the app. If it works, it means django found the secret key randomly generated.

Checklist

• Code review by me
• Hand tested by me
• I'm proud of my work
• Code review by reviewer
• Hand tested by reviewer
• CircleCi tests are passing
• Ready to merge

[Didayolo]

The CircleCI tests are failing

Error response from daemon: Container e1b459a9a774a3304902586c6bb054308ae2ac6c61a91d656a474a6b30233e3a is restarting, wait until the container is running