How to make an image from a Docker container - docker

Question

I'm following a tutorial to wrap a tool in a docker container.
In the linked tutorial page, step 2 describes how to create the container
$ docker run -ti ubuntu
and
root#70235f7726cf:/#
I install a number of libraries/programs
$ apt-get install wget build-essential zlib1g-dev libncurses5-dev
[...]
then
exit
Step 3 describes how the docker container is saved into an image but there is only the procedure to save it in a private repo and not in docker hub.
I did some research and the following is the command to push an image to a Docker repository in the Hub
$ docker push myusr/my-repo:mytoolv1
but since I did not save the image the push does not work.
The tutorial I'm following is missing some steps in between or maybe it is me that is missing some knowledge of Docker.

Answer 1

I think you might have some terms mixed up. You can't push containers to dockerhub, you can only push images.
To create a custom image you need a Dockerfile. Something like this:
FROM ubuntu:18.04
RUN apt update
RUN apt install -y wget build-essential zlib1g-dev libncurses5-dev
...
Then from the same folder build the custom image by running
docker build -t myusr/my-repo:mytoolv1 .
Ath thus point you can push the image to dockerhub using the command you tried:
docker push myusr/my-repo:mytoolv1

Answer 2

What is in your mind and is not correct, is that you think you can push container to your local repo, but in fact you are pushing the image.
I hope you know the difference between an image and a container, if not you can search about.
You can create a file called Dockerfile (with no extension and with this exact name) with the following contents:
FROM ubuntu:20.04
# to set time zone as you may encounter some unexpected stuck when selecting time zone during the build
ENV TZ=Asia/Tehran # search more about it in https://www.php.net/manual/en/timezones.php
RUN ln -snf /usr/share/zoneinfo/$TZ /etc/localtime && echo $TZ > /etc/timezone
RUN apt-get update && apt-get install -y wget build-essential zlib1g-dev libncurses5-dev
Now you should build your image:
docker build -t yourrepo/NAME:TAG_VERSION .
example:
docker build -t yourrepo/my_image:1.0.0 .
Now you can push it:
docker push yourrepo/my_image:1.0.0