docker-compose postgres volume changes ownership to gitlab-runner - postgresql

Question

I have a very strange problem, and cannot find why this happens. I installed gitlab-runner on my computer a several months ago, maybe a year and didn't use it anymore .
I have a docker-composer.yml:
version: '2'
services:
db:
image: postgres:9.5
#restart: always
environment:
POSTGRES_PASSWORD: password
volumes:
- ./storage:/var/lib/postgresql/data
I create ./storage, a ls gives me:
drwxrwxr-x 2 pierre-emmanuel pierre-emmanuel 4096 jui 31 23:33 storage
When I run docker-compose up -d , the user is changed to gitlab-runner ... I don't understand this AT ALL. It should have never happened.
Here is the ls I have:
drwx------ 19 gitlab-runner pierre-emmanuel 4096 jui 31 23:35 storage
Now I repeated this and with ps -aux | grep gitlab was able to see this:
gitlab-+ 2404 11.5 0.0 19704 3456 ? Ss 23:34 0:00 bash /usr/local/bin/docker-entrypoint.sh postgres
gitlab-+ 2514 6.0 0.0 19904 3912 ? S 23:35 0:00 initdb --username=postgres --pwfile=/dev/fd/63
gitlab-+ 2536 0.0 0.0 4280 712 ? S 23:35 0:00 sh -c "/usr/lib/postgresql/9.5/bin/postgres" --single -F -O -c search_path=pg_catalog -c exit_on_error=true template1 >/dev/null
gitlab-+ 2537 0.0 0.3 281888 28572 ? R 23:35 0:00 /usr/lib/postgresql/9.5/bin/postgres --single -F -O -c search_path=pg_catalog -c exit_on_error=true template1
Then after it stays like this until I call docker-composer stop:
gitlab-+ 2404 0.0 0.2 274528 23304 ? Ss 23:34 0:00 postgres
gitlab-+ 2618 0.0 0.0 274528 3780 ? Ss 23:35 0:00 postgres: checkpointer process
gitlab-+ 2619 0.0 0.0 274528 5400 ? Ss 23:35 0:00 postgres: writer process
gitlab-+ 2620 0.0 0.0 274528 3780 ? Ss 23:35 0:00 postgres: wal writer process
gitlab-+ 2621 0.0 0.0 274956 6252 ? Ss 23:35 0:00 postgres: autovacuum launcher process
gitlab-+ 2622 0.0 0.0 129512 2828 ? Ss 23:35 0:00 postgres: stats collector process
My own user and gitlab-runner do have the docker group, but I can't figure out why this is producing. Do you have any idea ?
EDIT: when I remove the app, remove the volume from docker-compose.yml and re-create the app, the user gitlab-runner is still used to run the container which contains postgres.

Answer 1

Processes and files are owned by numeric user and group IDs. There's a file, /etc/passwd, that maps between user names and user IDs; but the nature of Docker is that each container has its own isolated filesystem space, which means it has its own /etc/passwd file.
If you look at the Docker Hub postgresql page there is a link to the image's Dockerfile and you can see that includes a command
RUN useradd ... --uid=999 ... postgres
You should be able to verify that the user IDs on your local system and inside the container match up
grep gitlab-runner /etc/passwd
docker run --rm postgres:9.5 grep postgres /etc/passwd
I would expect both of these to show a uid of 999 in the third field. The files are owned by that uid, but it happens to translate to a different name inside and outside the container.