Feat: Allow passing environment variables in to container

[ghost]

In my usecase, I need to be able to configure some of the security environment variables when starting the localstack container. This PR passes an optional dictionary of extra settings when starting the container.

e.g.

localstack = pytest_localstack.patch_fixture(
    services={"s3"},
    container_env={
        "DISABLE_CORS_CHECKS": "1",
    },
)