docker build without a Dockerfile

I recently wanted to create an Ubuntu container image with some extra stuff installed, but I couldn’t be bothered to create a Dockerfile. It turns out that docker has a docker container commit command, so I used that. Here’s how.

First we use docker run with an ubuntu image, telling it to use apt-get to install our extra stuff. We use --cidfile which causes the container ID to be written to the specified file. Then use use that container ID to commit an image.

docker run --cidfile ubuntu-tmp.cid ubuntu \
    /bin/bash -c "export DEBIAN_FRONTEND=noninteractive; \
                    apt-get update && \
                    apt-get -y upgrade && \
                    apt-get -y install curl jq"
docker container commit $(<ubuntu-tmp.cid) ubuntu-tmp
rm ubuntu-tmp.cid

Unfortunately, the command we passed above becomes the default command for the new image, so we need another step to get rid of that:

docker run --cidfile ubuntu-extra.cid ubuntu-tmp \
    /bin/bash
docker container commit $(<ubuntu-extra.cid) ubuntu-extra
rm ubuntu-extra.cid

Now when we run it, it has our extra stuff installed:

% docker run -it ubuntu-extra
root@7b1fedeaf0ed:/# curl --version
curl 8.5.0 [etc.]
root@7b1fedeaf0ed:/# jq --version
jq-1.7

In hindsight, however, I’m gonna end up putting the snippet at the top in a shell script, aren’t I? So I might as well just use a Dockerfile. Still, this might be useful to someone.