Enable SSH connection in a container.

Dockerfile

FROM ubuntu:20.04

RUN apt-get update \
    && apt-get install -y --no-install-recommends \
        openssh-server \
    && mkdir -p /run/sshd \
    && echo "root:auth" | chpasswd \
    && sed -i 's/^#\(PermitRootLogin\) .*/\1 yes/' /etc/ssh/sshd_config \
    && sed -i 's/^\(UsePAM yes\)/# \1/' /etc/ssh/sshd_config

EXPOSE 22

CMD ["/usr/sbin/sshd", "-D"]