Add HOME env var

[hithwen]

Avoid:

tests/conftest.py:66: in dd_environment
    with docker_run(
/opt/hostedtoolcache/Python/3.8.7/x64/lib/python3.8/contextlib.py:113: in __enter__
    return next(self.gen)
../datadog_checks_dev/datadog_checks/dev/docker.py:208: in docker_run
    with environment_run(
/opt/hostedtoolcache/Python/3.8.7/x64/lib/python3.8/contextlib.py:113: in __enter__
    return next(self.gen)
../datadog_checks_dev/datadog_checks/dev/env.py:73: in environment_run
    condition()
tests/conftest.py:105: in __call__
    raise Exception(result.stderr)
E   Exception: WARNING: Error loading config file: .dockercfg: $HOME is not defined
- generated xml file: /home/vsts/work/1/s/vault/.junit/test-unit-py38-noauth.xml