Fix some typos in the new in-code Dockerfile documentation.
Fix some typos in the new in-code Dockerfile documentation.

These are trivial typos, addressing missing `)` and incorrect
terminology ("image" instead of the intended "container").

chipx86/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,chipx86/reviewboard