is docker changing ownership of files?

Question

Our deployment model is that we create containers on the fly with docker-java-api, some of these containers use heavily rocksdb database. The files of the DB are on the host, like:

ls -lrt /mnt/data/rocksdb
-rw-r--r-- 1 root root    8374 Nov 28 15:32 fileA
-rw-r--r-- 1 root root       0 Nov 28 15:32 fileB
-rw-r--r-- 1 root root      37 Nov 28 15:32 ....
-rw-r--r-- 1 root root      16 Nov 28 15:32 fileC
-rw-r--r-- 1 root root   19646 Nov 28 15:32 ..
-rw-r--r-- 1 root root   22500 Nov 28 15:32 .... etc

/mnt/data/rocksdb gets mounted onto a container with app that uses the DB heavily.

What I notice is that after starting the container, ownership of the files changes to:

ls -lrnt /mnt/data/rocksdbdata/
total 84092
-rw-r--r-- 1 999 999    8374 Nov 28 15:32 fileA
-rw-r--r-- 1 999 999       0 Nov 28 15:32 LOCK
-rw-r--r-- 1 999 999      37 Nov 28 15:32 fileB
-rw-r--r-- 1 999 999      16 Nov 28 15:32 fileC
-rw-r--r-- 1 999 999   19646 Nov 28 15:32 ...
-rw-r--r-- 1 999 999   22500 Nov 28 15:32 .....etc

User with these UID:GID (999:999) is docker.

Can you tell me why is this happening?

Answer 1

It needs a long explanation. Long story short, docker is changing the mounted file permissions because of access with root permissions.

For more details, please look at this answer;
Docker changes owner of local files mounted as volume