Long response times when validating

We're running the Docker Image of the validator badge. When trying to validate from an internal resource it takes 2+ minutes to get a response:

root@80b4f461309a:/validator# time curl http://localhost:8080/?url=http://my.host.domain:8041/my/path/swagger.json
[…]
real    2m7.294s
user    0m0.015s
sys     0m0.010s

Accessing the given url directly works without any issues:

root@80b4f461309a:/validator# curl http://my.host.domain:8041/my/path/swagger.json
{
  "swagger" : "2.0",
  "info" : {

Looks like it runs into timeout. The container has no Internet access and it responds quickly when not passing a url.

Any pointers to what's going on?

Reproduce the curl request from inside the Docker image, comparing validation of the internal Swagger URL with validation without a URL. Inspect the validator's URL-fetching and timeout behavior; done means the internal resource is handled without an unexplained two-minute delay, or a prompt and clear failure is returned.