How docker volume mounts differ

Question

Please look at the following output.

docker run -it --rm ireshmm/jenkins:lts-jdk11 ls -la /var/jenkins_home

Output:

total 40
drwxr-xr-x 3 jenkins jenkins  4096 Jan 22 16:20 .
drwxr-xr-x 1 root    root     4096 Jan 12 15:46 ..
-rw-r--r-- 1 jenkins jenkins  4683 Jan 22 16:20 copy_reference_file.log
drwxr-xr-x 2 jenkins jenkins 16384 Jan 22 16:20 plugins
-rw-rw-r-- 1 jenkins root     7152 Jan 12 15:42 tini_pub.gpg

When we mount a fresh new volume at /var/jenkins_home,

docker run -v random_volume:/var/jenkins_home -it --rm ireshmm/jenkins:lts-jdk11 ls -la /var/jenkins_home

Output:

total 40
drwxr-xr-x 3 jenkins jenkins  4096 Jan 22 16:24 .
drwxr-xr-x 1 root    root     4096 Jan 12 15:46 ..
-rw-r--r-- 1 jenkins jenkins  4683 Jan 22 16:24 copy_reference_file.log
drwxr-xr-x 2 jenkins jenkins 16384 Jan 22 16:24 plugins
-rw-rw-r-- 1 jenkins root     7152 Jan 12 15:42 tini_pub.gpg

Question : How do the files still exist? Shouldn't they be gone?

When we mount a empty directory on host,

mkdir /tmp/empty_dir && chmod 777 /tmp/empty_dir
docker run -v /tmp/empty_dir:/var/jenkins_home -it --rm ireshmm/jenkins:lts-jdk11 ls -la /var/jenkins_home

Output:

total 32
drwxrwxrwx 3    1001    1001  4096 Jan 22 16:28 .
drwxr-xr-x 1 root    root     4096 Jan 12 15:46 ..
-rw-r--r-- 1 jenkins jenkins  4683 Jan 22 16:28 copy_reference_file.log
drwxr-xr-x 2 jenkins jenkins 16384 Jan 22 16:28 plugins

Question : Why do the output differ from the previous output? What could be the reason?

Answer 1

This is described in the documentation :

If you start a container which creates a new volume, as above, and the container has files or directories in the directory to be mounted (such as /app/ above), the directory's contents are copied into the volume. The container then mounts and uses the volume, and other containers which use the volume also have access to the pre-populated content.

Bind mounts don't share this behavior.