Add support for the Dockerfile USER directive so that the container's
CMD/ENTRYPOINT runs as the configured user instead of root.
Changes:
- inject.rs: Add user field to Config struct, write /slim/user at
build time
- build.rs: Pass config.user through to inject()
- slim-init.sh: Read /slim/user and drop privileges via su before
executing the command. Numeric uids are resolved to usernames via
/etc/passwd (BusyBox su does not accept numeric args). When dropping
privileges, run as a child (not exec) so PID 1 stays root and can
poweroff after the command exits.
- test.sh: Add test_user verifying build-time CMD and --cmd override
both run as the configured user
Closes#7
marvin