Preparation for http proxy caching (#8281)

* always use http for apt repos

* pass possible proxy arguments to docker

* populate lower-case env variables with upper-case ones if not set otherwise

lib/functions/rootfs/distro-specific.sh

@@ -196,7 +196,7 @@ function create_sources_list_and_deploy_repo_key() {
 	fi
 	cat <<- EOF > "${basedir}"/etc/apt/sources.list.d/armbian.sources
 	Types: deb
-	URIs: https://${armbian_mirror}
+	URIs: http://${armbian_mirror}
 	Suites: $RELEASE
 	Components: ${components[*]}
 	Signed-By: ${APT_SIGNING_KEY_FILE}