Fix: First time developer setup

[NiceGuyIT]

While setting up the dev environment for the first time, I ran into some issues. This PR fixes .env.example to help future developers.

1. There was an error that the database doesn't exist. Adding POSTGRES_DB silenced the error. I don't know if this is absolutely required but it doesn't hurt.
2. The nginx container (pulled from Docker Hub) expects CERT_PUB_KEY and CERT_PRIV_KEY to be base64 encoded
3. While using the certs to setup a complete HTTPS environment, the frontend kept generating an error. Adding CORS_ORIGIN_WHITELIST helped along with the corresponding change in tacticalrmm-web (separate PR).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.85%. Comparing base (564aaaf) to head (2cf1c0d).
⚠️ Report is 457 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1677      +/-   ##
===========================================
- Coverage    80.93%   80.85%   -0.09%     
===========================================
  Files          136      136              
  Lines         9218     9228      +10     
===========================================
  Hits          7461     7461              
- Misses        1757     1767      +10     

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[sadnub]

Looks good to me