docker/ci: extract DOCKER_SERVER from DOCKER_IMAGE, and use for docker login
docker/ci: extract DOCKER_SERVER from DOCKER_IMAGE, and use for docker login

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