A small batch of updates

[Ramblurr]

Loving this project. It was exactly what I needed to build nextcloud-filedrop.

Here's a small batch of updates:

• We throw an error now when authentication fails, the error message otherwise is obscure and not obvious
• For development purposes I like to use a .env file rather than export the vars into my shell session. So I added python-dotenv as an optional dependency. If it exists the env vars will be populated from .env.
• Added a new field to the POST request: date from the date header of the email
• The Dockerfile was broken. The tagged base image doesn't exist anymore. I changed it to pin to 3.8-slim

[Ramblurr]

@allan-simon getting back round to this, sorry for the super long delay... things came up.

I've addressed your feedback!