docker/ci: create-docker-image.sh default to $arch/ubuntu as base image.
docker/ci: create-docker-image.sh default to $arch/ubuntu as base image.

$BASE is overrideable an environment variable too.

skydive-project/skydive,safchain/skydive,lebauce/skydive,lebauce/skydive,lebauce/skydive,skydive-project/skydive,skydive-project/skydive,lebauce/skydive,skydive-project/skydive,safchain/skydive,skydive-project/skydive,safchain/skydive,safchain/skydive,safchain/skydive,safchain/skydive,lebauce/skydive,safchain/skydive,lebauce/skydive,lebauce/skydive,skydive-project/skydive,lebauce/skydive,safchain/skydive,skydive-project/skydive