Docker: where is the newly created files under / stored? - docker

Question

I was running a docker container process with:
host$ docker run -it <image> /etc/bootstrap.sh -bash
Then inside of the container I created a file /a.txt:
container$ echo "abc" > /a.txt
container$ cat a.txt
abc
I noticed the filesystem type for / is none:
container$ df -h
Filesystem Size Used Avail Use% Mounted on
none 355G 19G 318G 6% /
tmpfs 1.9G 0 1.9G 0% /dev
...
The inspect command shows the volumes is null.
host$ docker inspect <image>
...
"Volumes": null,
...
After I exited the container process and restarted, the file disappeared. I wanted to understand:
1) what the root filesystem of the container process actually is;
2) how can I persist the newly created file?

Answer 1

Q: After I exited the container process and restarted, the file disappeared.
A: Data in a docker container is not persisted. That is you lost everything when that container gets restarted.
Q: What the root filesystem of the container process actually is?
A: Don't really understand this question but I assume you are asking about where is the root user's home directory? If it is, then root's home is at /root.
Q: How can I persist the newly created file?
A: If you are intending to keep the changes even after you restart the container then you will need to use docker's data volume.
See:
https://docs.docker.com/engine/tutorials/dockervolumes/
Essentially when you start the container, you can pass in the -v option to tell the container that you would like to map the directory from the host's file system to the container's directory.
That is by doing the example command below,
$ docker run -d -P --name web -v $(pwd):/root
You will you would like to map your current working directory to the container's /root directory. So everything gets written to the container's /root/ area gets reflected to your host's file system and it is persisted.