To step through this recipe, you will need a working Docker installation.

To have a fixed command executed by the container, use the ENTRYPOINT instruction. Use an array if the command is followed by arguments to be enforced:

FROM debian:stable-slim
RUN apt-get update -y && 
    apt-get install -y apache2 && 
    rm -rf /var/lib/apt/
EXPOSE 80
ENTRYPOINT ["/usr/sbin/apache2ctl", "-D", "FOREGROUND"]

To override the whole command at runtime, use the --entrypoint option:

$ docker run -it --rm --entrypoint /bin/sh httpd
# hostname
585dff032d21

To have a command that can be simply overridden with an argument, use the CMD instruction instead of ENTRYPOINT:

FROM debian:stable-slim
RUN apt-get update -y && 
    apt-get install -y apache2 && 
    rm -rf /var/lib/apt/
EXPOSE 80
CMD ["/usr/sbin/apache2ctl", "-D", "FOREGROUND"]

To override the command, simply give another command as an argument at runtime:

$ docker run -it --rm httpd /bin/sh
# hostname
cb1c6a7083ad

We can combine both instructions to have a more dynamic container. In this case, we want to obtain a container always executing /usr/sbin/apache2ctl, and by default starting the daemon in foreground, otherwise overridden by any argument at container launch time:

FROM debian:stable-slim
RUN apt-get update -y && 
    apt-get install -y apache2 && 
    rm -rf /var/lib/apt/
EXPOSE 80
CMD ["-D", "FOREGROUND"]
ENTRYPOINT ["/usr/sbin/apache2ctl"]

If this container is executed as is, nothing changes; apache2ctl gets executed with the -D FOREGROUND option.

However, it becomes a more useful container when giving it arguments, as it dynamically will add them to the apache2ctl command, replacing the original command specified by the CMD instruction:

$ docker run -it --rm httpd -v
Server version: Apache/2.4.10 (Debian)
Server built:   Sep 15 2016 20:44:43

We can interactively pass /usr/sbin/apache2ctl arguments without the need to override the entrypoint, for example, to propose alternatives Apache configuration files or options.