Error created when updating task from Flutter app

[sternj]

A task without a due date is given the string -, which is sent to the backend and is not able to be used by the CLI. Since tasks have default values in the taskDetails UI, perhaps these should be ignored. I'm testing a fix!

[sternj]

(by the way, I also cleaned up the Dockerfile-- I'll be making a more in-depth PR describing what I did and how it speeds up repeated builds)