Why Docker Creates Root‑Owned Files and How to Fix Permission Errors

Background

A batch‑processing job was deployed with Docker. After launch, other applications could not access the directories created by the Docker job because the files were owned by root.

Phenomenon

The job creates directories and files, but when another process tries to read or write them it receives a Permission denied error. Inspection shows the generated paths are owned by root even though the launch script was executed by a regular user.

Solution

Identify the Docker user flag

Docker runs containers as root by default. The -u / --user option of docker run allows specifying a UID (and optional GID) for the container process. sudo docker run --help Relevant excerpt:

-u, --user string   Username or UID (format: <name|uid>[:<group|gid>])

Obtain the host UID and GID

Use the id command or inspect /etc/passwd to find the numeric identifiers of the user that should own the files.

UID: id -u (e.g., 1002)

GID: id -g (e.g., 1002)

Alternatively, view the entry in /etc/passwd:

cat /etc/passwd