It is highly recommended that processes within containers do not run as root in order to prevent container breakouts; however, that goal is hard to maintain when attached volumes come into play. For example, Synology's DiskStation manages ownership of attachable volumes as regular UNIX users; however, its Docker integration doesn't allow for aligning the container's user with what is normally -u on the docker run command-line.

entrypoint-demoter solves this types of scenarios by allowing the container to momentarily run as root, but demote the initial sub-command to either match the ownership of a given path or be explicitly defined by UID/GID environment variables.

The command line arguments remaining after the options below are used to execute a sub-command with the demoted user ID (uid) and group ID (gid).

If executed as a non-root user, then this tool skips demoting entirely and just executes the sub-command with the current uid and gid.

• UID : if set, demotes the sub-command to run with the given user ID
• GID : if set, demotes the sub-command to run with the given group ID

• --match PATH : uses the ownership of the given path to determine a user and group ID
• --stdin-on-term MESSAGE : some applications prefer to be gracefully stopped with a command on stdin rather than handling SIGTERM, such as Minecraft servers. The MESSAGE and a newline will be written to the sub-command's stdin when a TERM signal is received.
• --debug : enables debug logging
• --version : show version and exit

The following shows how to use entrypoint-demoter in a Debian based Dockerfile. The entry.sh would be where your own entry point script/application would be specified.

ARG DEMOTER_VERSION=0.1.0
ARG DEMOTER_ARCH=amd64

ADD https://github.com/itzg/entrypoint-demoter/releases/download/${DEMOTER_VERSION}/entrypoint-demoter_${DEMOTER_VERSION}_linux_${DEMOTER_ARCH}.deb /usr/src

RUN dpkg -i /usr/src/entrypoint-demoter_${DEMOTER_VERSION}_linux_${DEMOTER_ARCH}.deb

ENTRYPOINT ["/usr/local/bin/entrypoint-demoter", "--match", "/data", "/entry.sh"]

The full context of this example can be seen in this Dockerfile

On a Linux system with Make and Docker installed invoke:

make test