Docker containers have become the de-facto industry standard.Docker builds often break, and a large amount of efforts are put into troubleshooting broken builds. Prior studies have evaluated the rate at which builds in large organizations fail. However, little is known about the frequency and fix effort of failures that occur in Docker builds of open-source projects. This paper provides a first attempt to present a preliminary study on 857,086 Docker builds from 3,828 open-source projects hosted on GitHub. Using the Docker builds data, we measure the frequency of broken builds and report their fix time. Furthermore, we explore the evolution of Docker build failures across time. Our findings help to characterize and understand Docker build failures and motivate the need for collecting more empirical evidence.