Difference between Docker Build and Docker Run

Question

If I want to run a python script in my container what is the point in having the RUN command, if I can pass in an argument at build along with running the script?

Each time I run the container I want x.py to be run on an ENV variable passed in during the build stage.

If I were to use Swarm, and the only goal was to run the x.py script, swarm would only be building nodes, rather than building and eventually running, since the CMD and ENTRYPOINT instructions only happen at run time.

Am I missing something?

Answer 1

The docker build command creates an immutable image. The docker run command creates a container that uses the image as a base filesystem, and other metadata from the image is used as defaults to run that image.

Each RUN line in a Dockerfile is used to add a layer to the image filesystem in docker. Docker actually performs that task in a temporary container, hence the selection of the confusing "run" term. The only thing preserved from that RUN command are the filesystem changes, running processes, changes to environment variables, shell settings like the current working directory, are all lost when the temporary container is cleaned up at the completion of the RUN command.

The ENTRYPOINT and CMD value are used to specify the default command to run when the container is started. When both are defined, the result is the value of the entrypoint is run with the value of the cmd appended as a command line argument. The value of CMD is easily overridden at the end of the docker run command line, so by using both you can get easy to reconfigure containers that run the same command with different user input parameters.

If the command you are trying to run needs to be performed every time the container starts, rather than being stored in the immutable image, then you need to perform that command in your ENTRYPOINT or CMD. This will add to the container startup time, so if the result of that command can be stored as a filesystem change and cached for all future containers being run, you want to make that setting in a RUN line.