What does creating a simple docker parent image actually do? - docker

Question

I'm following the instructions on Docker's website on building a parent image. I'm very new to Docker. I'm on a CentOS 7.5.
I ran the mkimage-yum.sh script suggested on the Docker website for CentOS. I didn't understand why the last line of the script, rm -rf "$target" was there because it seems to delete all the work done by the script. So I commented it out and it leaves a directory /tmp/mkimage-yum.sh.ahE8xx, which looks like a minimal linux image with the typical linux file structure (e.g. /usr/,/etc/)
In my home directory, I compiled the program,
main.c :
#include <stdio.h>
#include <stdlib.h>
int main(void){
printf("Hello Docker World!\n");
return 0;
}
Using gcc -static -static-libgcc -static-libstdc++ -o hello main.c, I compiled the code to a statically linked executable as prescribed in the docker webpage.
I created the Dockerfile,
e.g.
FROM scratch
ADD hello /
CMD ["/hello"]
I start up the dockerd server and in a separate terminal I run docker build --tag hello .
The output is :
Sending build context to Docker daemon 864.8 kB
Step 1/3 : FROM scratch
--->
Step 2/3 : ADD hello /
---> Using cache
---> a38d49d40e50
Step 3/3 : CMD /hello
---> Using cache
---> 3bcbb04c367f
Successfully built 3bcbb04c367f
Gee whiz, it looks like it worked! However, I still only see Dockerfile hello main.c in the directory I did this. Docker clearly thinks it did something, but what? It didn't create any new files.
Now I run docker run --rm hello and it outputs Hello Docker World!.
However, I get disconcerting errors from the dockerd server:
ERRO[502548] containerd: deleting container error=exit status 1: "container f336b3a5505879453b4f7a00c06acf274d0a5f8b3d260762273a2d7c0a846141 does not exist\none or more of the container deletions failed\n"
WARN[502548] f336b3a5505879453b4f7a00c06acf274d0a5f8b3d260762273a2d7c0a846141 cleanup: failed to unmount secrets: invalid argument
QUESTIONS :
What exactly did docker build --tag hello . do? I see no output from this.
What are the dockerd errors all about? Maybe looking for the docker image not created by docker build?
How does the mkimage-yum.sh fit into this? Why does it delete all the work that it does at the end.

Answer 1

When you run --rm with docker run, the container will run then delete itself. If you want to keep the container, remove the --rm from the command.
The command docker build will grab the dockerfile and create a local image, in you case, from scratch with the image name of hello.

Answer 2

You will not see any additional files created in your folder. To see the created image, run command docker images. Here you should be able to see your image built with tag hello.
When you run docker run <imagename> it starts up a container with the provided image. That's why you see your C program's output