A template for best practices in writing a docker image

docker-entrypoint.sh

#!/bin/bash
set -e

if [ "$1" == 'mycmd' ]; then
    
    echo "I do what is usual"
    # INIT?
    
    # EXECUTE CUSTOM SCRIPTS?    
    for f in /docker-entrypoint.d/*; do
	case "$f" in
		*.sh)     echo "$0: running $f"; . "$f" ;;
		*)        echo "$0: ignoring $f" ;;
	esac
	echo
    done
else
    echo "Executed requested command?"
    $1
fi

Dockerfile

#######################
## BUILD BASE

# Small image
FROM alpine:3.5
# or
FROM ubuntu:16.04
# LTS

MAINTAINER "myself"

# allow custom scripts to be executed at launch time
RUN mkdir /docker-entrypoint.d/

#######################
## ENVIRONMENT VARIABLES with VERSIONS or SETUP

ENV MYSOFTWV 1.0

# Set utf8
ENV LANG en_US.utf8

#######################
## INSTALL PACKAGE FROM PACKAGING SYSTEM

RUN apt/apk update && install && clean

#######################
## SUBPACKAGING and CONFIGURING

RUN pip install ...

RUN dpkg ...

#######################
## VOLUMES

VOLUME /var/lib/mysoftware

#######################
## ENTRYPOINT

COPY docker-entrypoint.sh /usr/local/bin/
# RUN ln -s usr/local/bin/docker-entrypoint.sh / # backwards compat
ENTRYPOINT ["docker-entrypoint.sh"]

#######################
## EXPOSE ports and launch command

EXPOSE 5432
CMD ["mycmd"]