Dockerfile basics - Building Docker Images

Building Images: The Dockerfile Basics

The default filename is Dockerfile, but you can specify with docker build -f <some-dockerfile>.
One of the reasons to build containers FROM Debian, Ubuntu, alpine, etc is that we can use their package managers to install new software.

# FROM tells which image we want to use as a base.
# a good reason to use debian, ubuntu, fedora, centos, etc. is to be
# able to use their package managers (`apt`, `yum`, etc.)
FROM debian:stretch-slim

# ENV is used to set environment variables
# that's the preferred way to inject key/value into a container
ENV NGINX_VERSION 1.13.6-1~stretch

# RUN is used to execute shell commands inside the container
# pro-tip:  use && to chain multiple relative commands in order avoid
#           creating multiple unnecessary layers (saves time and space)
RUN apt-get update && apt-get install -y something...

# It's also a good practice to spit logs in stdout/stderr
# The line below forwards request and error logs to docker log collector
RUN ln -sf /dev/stdout /var/log/nginx/access.log \
    && ln -sf /dev/stderr /var/log/nginx/error.log

# EXPOSE sets the port to be exposed (by default there's no port being exposed)
# you still need to use -p or -P to open/forward these ports on host
EXPOSE 80 443

# CMD required. It's the final command that will be run everytime you
# launch a new container from the image or start a stopped one.
# only one CMD is allowed, so if there are multiple, last one wins

Buildiung Images: Running Docker Builds

# build a new image from Dockerfile present in the current directory
# and tag it as 'customnginx'
docker image build -t customnginx .

Building Images: Extending Official Images

# extending an existing official image from Docker Hub

FROM nginx:latest
# it's hightly recommended to always pin versions for anything beyond dev/learn

WORKDIR /usr/share/nginx/html
# change working directory to root of nginx webhost,
# it's a preferred method than using 'RUN cd /some/path'

# COPY <source> <destination>
COPY index.html index.html
# some notes about COPY:
# - <source> path must be inside the context of the build (you can't use '..')
# - if <source> is a directory, the entire contents of it is copied
# - for more info check <https://docs.docker.com/engine/reference/builder/#copy>

# no need to specify EXPOSE or CMD as they're already set in the FROM image

Dockerfile Best Practice

Things you change the most should be at the bottom of the file.

Why? Because when you change a layer, the consecutive ones will need to be rebuilt.