Single word Adjective or noun for a person who always toes the line (to irritating level)

Difficult to find a condensation of meaning to one word. Three possibilities are:

Running-dog = one who blindly follows someone else's orders

Lackey = a servile follower

Flunkey = a person who does unimportant work or who has few or no important responsibilities and shows too much respect toward his or her employer:

All from Merriam Webster dictionary. The choice will depend on context.

If the person is merely compliant to rules rather than to a person, they might also be described as a rule-follower.