It's clearly a proxy issue: docker proxies https connections to the wrong place. Bear in mind that docker proxy settings may be different from the operating system (and curl) ones. Here's how I managed to solve the issue:
First of all, find out where are you proxying your docker https requests:
# docker info | grep Proxy
Http Proxy: http://<my.proxy.server>:8080
Https Proxy: https://<my.proxy.server>:8080
No Proxy: localhost,127.0.0.1
and double check your https settings.
In my case, I realized that the "Https proxy" was set to https://... instead of http://..., so I corrected it in /etc/sysconfig/docker file (I'm using RHEL7) and, after a docker restart with:
# systemctl restart docker
the proxy variable shows up succesfully updated:
# docker info | grep Proxy
Http Proxy: http://<my.proxy.server>:8080
Https Proxy: http://<my.proxy.server>:8080
No Proxy: localhost,127.0.0.1
and everything works fine :-)