Understanding Dockerfile CMD/ENTRYPOINT

Question

I'm new to Docker. Trying to build small image with Transmission. Here is my Dockerfile :

#base image
FROM alpine:latest

#install Transmission
RUN apk update
RUN apk add transmission-daemon

#expose port
EXPOSE 9091

#start app
CMD ["/usr/bin/transmission-daemon"]

Then I start container:

docker run transmission

and it immediately quits. I supposed that it will stay running, as transmission-daemon should stay running. I tried ENTRYPOINT also, but result is the same. However, next version works as expected:

ENTRYPOINT ["/usr/bin/transmission-daemon"]
CMD ["-h"]

It runs, show Transmission help and quits. What I am missing about how Docker runs apps inside containers?

Answer 1

Docker keeps a container running as long as the process which the container starts is active. If your container starts a daemon when it runs, then the daemon start script is the process Docker watches. When that completes, the container exits - because Docker isn't watching the background process the script spawns.

Typically your CMD or ENTRYPOINT will run the interactive process rather than the daemonized version, and you let Docker take care of putting the container in the background with docker run -d . (The actual difference between CMD and ENTRYPOINT is about giving users flexibility to run containers from your image in different ways ).

It's worth checking the Docker Hub if you're looking at running an established app in a container. There are a bunch of Transmission images on Docker Hub which you can use directly, or check out their Dockerfiles to see how the image is built.