Bash: sudo: command not found - bash

Question

I have an up and running containers and I wish to execute a database backup. Apparently, a simple command from the docker such as: sudo mkdir new_folder result in: bash: sudo: command not found
What have I tried (on an intuitive level) I accessed one of the running container with docker exec -i -t 434a38fedd69/bin/bash and RUN
apt-get update
apt-get install sudo
when exit back to docker and tried to perform sudo mkdir new_folder but I got the same message bash: sudo: command not found
Baresp#adhg MINGW64 /c/Program Files/Docker Toolbox/postgre
$ mkdir new_folder
mkdir: cannot create directory ‘new_folder’: Permission denied
Baresp#adhg MINGW64 /c/Program Files/Docker Toolbox/postgre
$ sudo mkdir new_folder
bash: sudo: command not found
BTW, I'm not sure if this is relevant but the docker-compose file I was using is:
version: '2'
services:
postgres:
image: postgres
environment:
POSTGRES_USER: postgres
POSTGRES_PASSWORD: changeme
PGDATA: /data/postgres
volumes:
- /data/postgres:/data/postgres
ports:
- "5432:5432"
networks:
- postgres
restart: unless-stopped
container_name: xx_postgres
pgadmin:
links:
- postgres:postgres
image: fenglc/pgadmin4
volumes:
- /data/pgadmin:/root/.pgadmin
ports:
- "5050:5050"
networks:
- postgres
restart: unless-stopped
container_name: xx_pgadmin
networks:
postgres:
driver: bridge

Answer 1

First, nothing you do in a docker exec is persistent outside of that particular running container (copy of the image), so if you want future containers run from that image to include sudo, those apt-get commands need to go into the Dockerfile that builds the image. Which, since you're using docker-compose, would require you to first make a Dockerfile and specify its location in the YAML.
Second, what do you mean "exit back to docker"? Nothing you do inside a container is going to have any effect on the system that Docker itself is running on, but it looks like you're running software install commands inside a Docker container and then expecting that to result in the newly-installed software being available outside the container on the Windows system that is running Docker.

Answer 2

To do a backup of the postgres database in the container, you first have to enter the container (similar to how you do it):
docker exec -it postgres bash
(substitude postgres with the real container name you get from docker-compose ps)
Now you are in the container as root. That means, you don't need sudo for anything. Next create your backup folder:
mkdir /tmp/backup
Now run the backup command, from a quick Google I found the following (you might know better):
pg_dumpall > /tmp/backup/filename
Then exit the shell within the container by typing exit. From your host system run the following to copy the backup file out of the container:
docker cp postgres:/tmp/backup/filename .
(postgres is your container name again)