docker: fix wrong state of example

[atline]

examples/docker: pytest -s --lg-env env.yaml test_shell.py will print next error:

______________________ ERROR at setup of test_shell ______________________

target = Target(name='main', env=Environment(config_file='env.yaml'))

    @pytest.fixture(scope='session')
    def command(target):
        strategy = target.get_driver('DockerStrategy')
>       strategy.transition("shell")

conftest.py:6:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
../../labgrid/step.py:215: in wrapper
    _result = func(*_args, **_kwargs)
../../labgrid/strategy/dockerstrategy.py:39: in transition
    status = Status[status]
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

cls = <enum 'Status'>, name = 'shell'

    def __getitem__(cls, name):
>       return cls._member_map_[name]
E       KeyError: 'shell'

/usr/lib/python3.8/enum.py:387: KeyError

This is because there is no shell state in dockerstrategy, it should be accessible.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (342dc90) 63.0% compared to head (e6fa794) 63.0%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1276     +/-   ##
========================================
- Coverage    63.0%   63.0%   -0.1%     
========================================
  Files         160     160             
  Lines       11859   11859             
========================================
- Hits         7482    7481      -1     
- Misses       4377    4378      +1     

see 1 file with indirect coverage changes

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