Explicitly install apt-transport-https and ca-certificates

See merge request !4

Raspbian buster is old enough that this isn't included by default, but it looks
like the mirror redirector now started redirecting to HTTPS mirrors. Thus,
install it so this has a chance to actually work.

This avoids importing it from the gitlab-runner system.

Implement CI

See merge request !2

And remove it afterwards.

Add gitlab-ci

See merge request !1