Dockerfile improvements.

- Use CMD instead of ENTRYPOINT to launch sshd (easier to overwrite when
using locally).
- Expose port 22.

Dockerfile

@@ -135,5 +135,6 @@ RUN curl --verbose https://www.mongodb.org/static/pgp/server-4.4.asc | apt-key a
     rm /etc/ssh/ssh_host_* && \
     rm -rf /tmp/* /var/tmp/* /var/lib/apt/lists/* /var/cache/apt/archives/*
 VOLUME /run/sshd
-ENTRYPOINT ["/usr/sbin/sshd", "-D"]
+EXPOSE 22
+CMD ["/usr/sbin/sshd", "-D"]
 HEALTHCHECK CMD nc -vzw3 localhost 22 || exit 1