Add document for running tests on linux using docker

[younata]

closes #374.

Adds documentation for one way to run the linux tests while using a Mac.

[jessesquires]

👏🏼

[younata]

SwiftLint won't run on this because I didn't change any .swift files, and I can't merge until that runs. The repository is also set up so that I can't even manually merge this in. @jessesquires as an owner, you should be able to override that and merge that, so I'll let you do. (can you up my privileges to owner?)

[jessesquires]

SwiftLint won't run on this because I didn't change any .swift files, and I can't merge until that runs.

ugh. yeah I would rather run Swiftlint via Danger, which doesn't have this problem. 😕

The repository is also set up so that I can't even manually merge this in. @jessesquires as an owner, you should be able to override that and merge that, so I'll let you do. (can you up my privileges to owner?)

Just made you an admin. Same for Quick. 👍🏼