I just downloaded a Ubuntu docker image, and now I'm confused. If Docker shares a common kernel among the containers, what's exactly inside an Ubuntu image? Which kernel is running?
First of all ask yourself what is Ubuntu ?
Ubuntu like CentOS, Arch and may others are Linux based distribution (distro). A distro is a operating system/software collection/packaging/philosophy based upon the Linux Kernel.
An Ubuntu docker image is the Software collection etc running in a container. It uses the host's Linux kernel.