If you have a custom CA and want your docker images to trust you certificates, you have to include the ca-certs in you images. This is different depending on the distribution of the base-image.

Debian Based

For Debian add these lines to your Dockerfile and provide the expected files.

FROM debian

ADD certs/my-cert.cer /usr/local/share/ca-certificates/my-cert.crt
RUN chmod 644 /usr/local/share/ca-certificates/*
RUN update-ca-certificates

...  

It is important that your cert needs to have the filetype .crt. Otherwise it will not be added.