The USER instruction allows you to set the current user (and group) for all of the instructions that follow in the Dockerfile, and for the containers that are run from the built image. The syntax for the USER instruction is as follows:
# User instruction syntax
USER <user>[:<group>] or
USER <UID>[:<GID>]
If a named user (or group) is provided as parameters to the USER instruction, that user (and group) must already exist in the passwd file (or group file) of the system, or a build error will occur. If you provide the UID (or GID) as the parameter to the USER command, the check to see whether the user (or group) exists is not performed. Consider the following Dockerfile:
# USER instruction Dockerfile for Docker Quick Start
FROM alpine
LABEL maintainer="Earl Waud <[email protected]>"
RUN id
USER games:games
run id
CMD ["...