Pre-pull images in Docker in Docker (dind) - docker

Question

I am running a docker-in-docker container that always uses the same few images.
I would like to pre-pull those in my dind container so I don't have to pull them at startup.
How would I be able to achieve this?
I was thinking of building my own dind image along the lines of
FROM docker:18.06.1-ce-dind
RUN apk update && \
apk upgrade && \
apk add bash
RUN docker pull pre-pulled-image:1.0.0
Obviously above Dockerfile will not build because docker is not running during the build, but it should give an idea of what I'd like to achieve.

Answer 1

You can't do this.
If you look at the docker:dind Dockerfile it contains a declaration
VOLUME /var/lib/docker
That makes it impossible to create a derived image with any different content in that directory tree. (This is the same reason you can't create a mysql or postgresql image with prepopulated data.)

Answer 2

Here is some way to achieve this.
The idea is to save images that you want to use, and then to import this images during the container startup process.
For example, you can use a folder images where to store your images.
And you have to use a customized entrypoint script that will import the images.
So the Dockerfile will be :
FROM docker:19.03-dind
RUN apk add --update --no-cache bash tini
COPY ./images /images
COPY ./entrypoint.sh /entrypoint.sh
ENTRYPOINT ["tini", "--", "/entrypoint.sh"]
(tini is necessary to load images in background)
And the entrypoint.sh script :
#!/bin/bash
# Turn on bash's job control
set -m
# Start docker service in background
/usr/local/bin/dockerd-entrypoint.sh &
# Wait that the docker service is up
while ! docker info; do
echo "Waiting docker..."
sleep 3
done
# Import pre-installed images
for file in /images/*.tar; do
docker load <$file
done
# Bring docker service back to foreground
fg %1
FYI, an example on how to save an image :
docker pull instrumentisto/nmap
docker save instrumentisto/nmap >images/nmap.tar