Update "http://" links in Dockerfile to "https://"

[iakkus]

The Dockerfile contains URLs to http://ports.ubuntu.com and http://cdn.kernel.org.

Although in a browser (e.g., Firefox) these URLs are automatically upgraded to use https, in the docker build process, this may not be the case. The servers themselves may redirect the request (in this case, only http://cdn.kernel.org), and sometimes not (http://ports.ubuntu.com). Even with the redirection from the server side, it is still not as secure as using directly https (i.e., a redirection may be hijacked in the network and modified).

As a result, these downloads are not secure.

Replacing the http:// URLs with https:// equivalents does not change the build process.

Please consider making the update.