I have installed a nginx docker image. But in the vm template i am unable to find any registered kernal image in CPU&OS tab.
Make sure you download a kernel image from the marketplace. By default OpenNebula has a datastore with ID 1 that holds file type images. Import a kernel appliance from the marketplace, like this one and it should appear afterwards in the datastore 1. Once it is there it should be listed in the CPU & OS tab.