docker_container: fix env_file option

[felixfontein]

SUMMARY

The env_file option was ignored because of a snafu handling options with not_a_container_option=True. Unfortunately the env_file tests weren't really testing what happens, so they were happily passing...

Fixes #451.

ISSUE TYPE

• Bugfix Pull Request

COMPONENT NAME

docker_container

[gotmax23]

This changes as are LGTM, but I have one suggestion.

The docs say:

- env_file
        Path to a file, present on the target, containing environment variables `FOO=BAR'.
        If variable also present in `env', then the `env' value will override.
        [Default: (null)]
        type: path

Perhaps it's also worthwhile to ensure that the overriding works properly (e.g. add a test with env_file: "{{ remote_tmp_dir }}/env-file" and env: {TEST3: other_val3} and make sure the value is other_val)?

[github-actions]

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This PR has been merged and your docs changes will be incorporated when they are next published.

[felixfontein]

@gotmax23 thanks for reviewing this!