Feature - Private Docker registries

Description

Adds support for private docker registries.

Changes

• Settings are immediately written to disk at startup (old bug)
• The security agent uses the correct key to find the authentication database (old bug)
• Moved integration tests around somewhat for a discarded test

Additions

• A model class for authentication configuration objects
• Fields in Assignment
• Getter in Job
• The Docker runner sends these to the Docker API
• Documentation

Test and Review

Closes #247 (closed)

To be filled in by the reviewers

• All of the methods are commented to expectation

• The methods are tested to satisfaction

• There are no unnecessary files present in the MR

• The continuous integration has no problems with the MR

• The MR is filled in as requested (including labels, milestones, and reviewers)

• The documentation is up-to-date

• All nullable parameters are marked as such