How do I add a CA root certificate inside a docker image?

Question

I am running an ASP.NET Core 1.1 Web API in a Docker 1.13.1 container on Ubuntu 14.04.

When the code attempts to retrieve some data from an HTTPS server, I get this

Answer 1

It's also worth noting that this definitely needs to use the .crt extension. I initially tried this with a .pem cert file (I thought they were interchangeable, so others might also), which is not linked by update-ca-certificates.