Login breaks Docker config

[pmeinhardt]

Executing pierone login breaks ~/.docker/config.json.

In particular, it sets an empty credHelper:

{
  "auths": {
    "https://pierone.stups.zalan.do": {
      "auth": "…",
      "email": "no-mail-required@example.org"
    }
  },
  "credHelpers": {
    "pierone.stups.zalan.do": ""
  }
}

After pierone login, on attempting to docker pull an image, it will fail:

Error response from daemon: Get https://pierone.stups.zalan.do/v2/…/manifests/latest: no basic auth credentials

Removing the empty credHelpers entry for "pierone.stups.zalan.do" fixes the problem.

I am using:

$ docker --version
Docker version 19.03.5, build 633a0ea

[aermakov-zalando]

Fixed in #93.