Java and custom UID mapping in (Docker) containers

print_user_homes.md

cat PrintUserHome.java
public class PrintUserHome {
    public static void main(String[] args) {
        System.out.println("User home: '" + System.getProperty("user.home") + "'");
    }
}

docker run -v `pwd`:/w -w /w --rm -it openjdk:11-slim sh -c 'javac PrintUserHome.java; java PrintUserHome'
User home: '/root'

docker run -v `pwd`:/w -w /w --rm -it --user=$(id -u ${USER}):$(id -g ${USER}) openjdk:11-slim sh -c 'javac PrintUserHome.java; java PrintUserHome'
User home: '?'

PrintUserHome.java

public class PrintUserHome {
    public static void main(String[] args) {
        System.out.println("User home: '" + System.getProperty("user.home") + "'");
    }
}

See also https://medium.com/redbubble/running-a-docker-container-as-a-non-root-user-7d2e00f8ee15

Your user will be $HOME-less
Your user will be nameless, too