In order to detect the base image, you would have to clearly specify the path to the dockerfile from the settings page.
Navigate to the project settings page > Dockerfile and configure your path accordingly. Please look at the screenshot below.
For eg., the path for dockerfile for this repository is https://github.com/CognitiveScale/cortex-cli/blob/develop/src/commands/assets/templates/skill/daemon/Dockerfile, which needs to be mentioned in the Path field