A docker container runs on an isolated file system. This custom file system is provided by a docker image. Because the image includes the container’s file system, it must have everything needed to run an application, all the dependencies, configurations, scripts, binaries, etc. The image also includes other configuration elements for the container, such as environment variables, a default command to run, and other metadata. A docker file must be built to create a docker container image. Additionally, the docker itself must be installed on the system before building the docker image.