Fantastic docstring!  See my comment above,   I think this would work better if you move the sub-subtest sections into the sub-subtest docstrings.  Then the main docstring here can just be like:

    A. Run a container using invalid charactor in "docker run [OPTIONS] IMAGE [COMMAND] [ARG...] "
    B. Execute docker run command, different subtest has different subargs and subargs order.
    C. Check test result
    D. Clean the test environment and remove the container generated in testing.

That'll make keeping the sub-subtest docs updated along with the code really easy.