harbor: Adding new Registry Endpoint (docker-hub) results in `Failed to ping endpoint` error on Photon OS 3.0

Expected behavior and actual behavior: Adding Docker Hub as avnew Registry Endpoint should be successful when running Harbor on Photon OS 3.0.

Instead, Harbor throws an error: Failed to ping endpoint.

core.log:

Sep 23 16:58:07 172.18.0.1 core[308]: 2019-09-23T16:58:07Z [ERROR] [/replication/adapter/native/adapter.go:143]: failed to ping registry https://registry-1.docker.io: Head https://registry-1.docker.io/v2/: Get https://registry-1.docker.io/v2/: dial tcp: lookup registry-1.docker.io on 127.0.0.11:53: read udp 127.0.0.1:41081->127.0.0.11:53: i/o timeout

cURL does not work either in the harbor-core container:

$ docker exec -it harbor-core curl --head https://registry-1.docker.io/v2/
curl: (6) Could not resolve host: registry-1.docker.io

cURL works okay if using the harbor-core image without docker-compose:

$ docker run --rm -it --network harbor_harbor --entrypoint bash goharbor/harbor-core:v1.9.0 
$ curl --head https://registry-1.docker.io/v2/
HTTP/1.1 401 Unauthorized
Content-Type: application/json
Docker-Distribution-Api-Version: registry/2.0
Www-Authenticate: Bearer realm="https://auth.docker.io/token",service="registry.docker.io"
Date: Mon, 23 Sep 2019 17:42:55 GMT
Content-Length: 87
Strict-Transport-Security: max-age=31536000

Steps to reproduce the problem:

1. Install Photon OS 3.0 from OVA
2. tdnf install docker tar gawk
3. Install docker-compose
4. Install Harbor using the online installer from the releases page on GitHub
5. Add a new Registry Endpoint using the docker-hub provider
6. Error shows up in the GUI and in core.log

Versions:

• harbor version: 1.8.3 and 1.9.0
• docker engine version: 18.06.2-ce
• docker-compose version: 1.24.1

core.log harbor.yml.txt