Some VPS doesn't have usermod

[certaintls]

AWS Debian 10 AMI doesn't have usermod. See https://github.com/certaintls/build-VPN-server/runs/4679434635?check_suite_focus=true

err: bash: line 2: usermod: command not found
out: 
out: ================================================================================
out: 
out: To run Docker as a non-privileged user, consider setting up the
out: Docker daemon in rootless mode for your user:
out: 
out:     dockerd-rootless-setuptool.sh install
out: 
out: Visit https://docs.docker.com/go/rootless/ to learn about rootless mode.
out: 
out: 
out: To run the Docker daemon as a fully privileged service, but granting non-root
out: users access, refer to https://docs.docker.com/go/daemon-access/
out: 
out: WARNING: Access to the remote API on a privileged Docker daemon is equivalent
out:          to root access on the host. Refer to the 'Docker daemon attack surface'
out:          documentation for details: https://docs.docker.com/go/attack-surface/
out: 
out: ================================================================================
out: 
err: Got permission denied while trying to connect to the Docker daemon socket at unix:///var/run/docker.sock: Post "http://%2Fvar%2Frun%2Fdocker.sock/v1.24/containers/gost-ss-kcp/stop": dial unix /var/run/docker.sock: connect: permission denied
err: Got permission denied while trying to connect to the Docker daemon socket at unix:///var/run/docker.sock: Post "http://%2Fvar%2Frun%2Fdocker.sock/v1.24/containers/gost-ss/stop": dial unix /var/run/docker.sock: connect: permission denied
err: docker: Got permission denied while trying to connect to the Docker daemon socket at unix:///var/run/docker.sock: Post "http://%2Fvar%2Frun%2Fdocker.sock/v1.24/containers/create?name=gost-ss-kcp": dial unix /var/run/docker.sock: connect: permission denied.
err: See 'docker run --help'.
err: docker: Got permission denied while trying to connect to the Docker daemon socket at unix:///var/run/docker.sock: Post "http://%2Fvar%2Frun%2Fdocker.sock/v1.24/containers/create?name=gost-ss": dial unix /var/run/docker.sock: connect: permission denied.
err: See 'docker run --help'.
20***/01/01 13:20:40 Process exited with status 126