The CMD instruction is used to define the default action taken when containers are run from images built with their Dockerfile. While it is possible to include more than one CMD instruction in a Dockerfile, only the last one will be significant. Essentially, the final CMD instruction provides the default action for the image. This allows you to either override or use the CMD in the image used in the FROM instruction of your Dockerfile. Here is an example where a trivial Dockerfile does not contain a CMD instruction and relies on the one found in the ubuntu image used in the FROM instruction:
You can see from the output of the history command that the ubuntu image includes the CMD ["/bin/bash"] instruction. You will also see that our Dockerfile does not have its own CMD instruction. When we run the container, the default action is to run "/bin...