Default to using https mirror

[collinanderson]

No idea if this would have would have unintended side-effects. But I saw that the url accepts https so I thought I'd propose using https.

Just a random drive-by pull request from someone who pretty much doesn't use docker at all. Feel free to close if it's a bad idea.

[tianon]

This would definely affect the ability to provide a transparent cache in front of Alpine image builds to speed up their downloads, but in Alpine those files are typically smaller so that's not quite as compelling as in something like Debian.