Implement non-interactive mode

[MatthewL246]

Resolves #3

Changes:

I implemented the non-interactive mode, which can be configured using environment variables or a .env file and is now the default unless the interactive flag is passed. I also added an optional Dockerfile and implemented parsing the Nintendo DER certificate directly, which removes the need to manually convert it to PEM first

• [x] I have read and agreed to the Code of Conduct.
• [x] I have read and complied with the contributing guidelines.
• [x] What I'm implementing was an approved issue.
• [x] I have tested all of my changes.

I have tested and confirmed that using an SSSL-patched certificate created from the DER works as expected on my Wii U with Inkay disabled.