Docker 101: Override Container Entrypoint to Run an Interactive Shell
Premium Challenge
Upgrade your membership to unlock this and all other premium materials.
One of the application types that Docker allows to containerize is CLI tools. For instance, httpie, a user-friendly command-line HTTP client, has a popular containerized version alpine/httpie.
Running the alpine/httpie image with no arguments prints httpie's help message:
docker run alpine/httpie
Running the alpine/httpie image with a single URL argument makes an HTTP GET request to it and prints the response:
docker run alpine/httpie https://github.com
But what if you wanted to launch a shell in the alpine/httpie image instead of running the httpie binary? It's a totally valid requirement when you want to explore the contents of the container.
Can you do it?
Hint 1: CMD vs. ENTRYPOINT
The only mandatory argument of the docker run command is the image name. When invoked in its shortest form (e.g., docker run alpine/httpie), Docker launches the container's default command, which is in turn a combination of two image's attributes: Cmd and Entrypoint.
Run the following command and locate them:
docker image inspect alpine/httpie
Hint 2: Overriding container's entrypoint
The alpine/httpie image has the entrypoint set to the http binary (i.e., the HTTP build of the httpie client). It means that executing docker run alpine/httpie with additional arguments will pass them as extra arguments to the containerized http command. This is why running the following two commands produces the same results:
# no prior httpie installation required
docker run alpine/httpie https://labs.iximiuz.com
# requires prior httpie installation
http https://labs.iximiuz.com
But there should be a way to override the entrypoint of a container. Look for the relevant part in the docker run --help output.
Hint 3: Running an interactive shell container
If you're not sure how to launch an interactive shell in a container, try solving these challenges first: