How do I make a comment in a Dockerfile?

You can use # at the beginning of a line to start a comment (whitespaces before # are allowed):

# do some stuff
RUN apt-get update \
    # install some packages
    && apt-get install -y cron

#‘s in the middle of a string are passed to the command itself, e.g.:

RUN echo 'we are running some # of cool things'