How do I create a Docker container which just waits to be shelled into?

Question

I'm testing a Docker container for development. And at the moment I just want to provision it with some libraries. After which I want to connect to it and test that I have all the libraries I need.

At the moment, my Dockerfile is just

FROM ubuntu:18.04

RUN apt update
RUN apt install -y git curl cmake libx11-dev libglu1-mesa-dev libxrandr-dev libxinerama-dev libxcursor-dev libxi-dev zlib1g-dev libasound2-dev libgtk2.0-dev libjack-jackd2-dev jq

But once it finishes building it terminates.

What CMD should I put at the end to keep it alive so I can shell in from another terminal?

Answer 1

Disclaimer: This is very much an anti-pattern since it looks like you are trying to create something similar to a VM, or at the very least a pet that you modify in place rather than making all your changes via code and redeploying a new container for each change.

That disclaimer aside, you just need a command that will hang indefinitely. The most common one I've seen in a tail -f /dev/null, e.g.:

FROM ubuntu:18.04

RUN apt update \
 && apt install -y \
      cmake \
      curl \
      git \
      jq \
      libasound2-dev \
      libglu1-mesa-dev \
      libgtk2.0-dev \
      libjack-jackd2-dev \
      libx11-dev \
      libxcursor-dev \
      libxinerama-dev \
      libxi-dev \
      libxrandr-dev \
      zlib1g-dev

CMD [ "tail", "-f", "/dev/null" ]

I've also rearranged the Dockerfile to merge the update and install commands, this is a best practice for Dockerfile's since it avoids having a stale apt update command used from the cache when you change the apt install command months later. Also, putting each package on a separate line is better for version control to see what changed if you add a single new package.

When using this, you would docker run -d --name your_container your_image to run it in the background, and then docker exec -it your_container /bin/bash to open a shell.