Single word Adjective or noun for a person who always toes the line (to irritating level)
Solution 1:
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.