3.0 Common Dockerfile commands

https://docs.docker.com/engine/reference/builder/ describes the commands one can use in a Dockerfile. Typically, the commands commonly used are:

• FROM: Initialises build and sets the base image. Generally considered a good idea to take official, LTS images based on Alpine Linux. Also, important to lock down and specify the version in the image tag.
• RUN: Runs a shell command in a new layer
• CMD: Provides defaults (executable or params to executable) for a container. A CMD overrides any preceding CMD layers. Executable can be omitted if there is an ENTRYPOINT. CMD is different from RUN, since RUN actually runs and creates a layer whereas CMD gets run during container run
• EXPOSE: Tells Docker that a container is listening on that port. It does not open the port between the host and the container. For that use the -p or -P option in docker run.
• ENV: sets environment key value pairs. These values can be changed during docker run.
• ADD: Copies new files, directories and remote file URLs from source to destination. Note that the directory is not copied but the contents.
• COPY: Is simpler than ADD.
• ENTRYPOINT: Configures a container to run as an executable.
• VOLUME: Creates a mount point to share files from the host system. However, the filesystem from the host machine needs to be mounted at this point when doing a docker run.
• USER: Changes the user (and group) for subsequent instructions.
• WORKDIR: Sets the working directory. Once set, subsequent commands can take relative paths.
• ARG: Used to pass in variables during build time

3.1 Container view to the outside world

You would have noticed that a running container interacts with the Host OS via port EXPOSE, filesystem mounted via VOLUME , takes in environment variables via ENV, so let's look at a few Dockerfiles.

FROM debian:jessie
MAINTAINER NGINX Docker Maintainers "[email protected]"
ENV NGINX_VERSION 1.11.7-1~jessie
RUN apt-key adv --keyserver hkp://pgp.mit.edu:80 --recv-keys 573BFD6B3D8FBC641079A6ABABF5BD827BD9BF62 \
     && echo "deb http://nginx.org/packages/mainline/debian/ jessie nginx" >> /etc/apt/sources.list \
     && apt-get update \
     && apt-get install --no-install-recommends --no-install-suggests -y \
                               ca-certificates \
                               nginx=${NGINX_VERSION} \
                               nginx-module-xslt \
                               nginx-module-geoip \
                               nginx-module-image-filter \
                               nginx-module-perl \
                               nginx-module-njs \
                               gettext-base \
     && rm -rf /var/lib/apt/lists/*
# forward 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 80 443
CMD [ "nginx", "-g", "daemon off;" ]

Exercise 3.1.1

Start from a Base image of your choice and do the following (I have taken example of a node app, you are free to do the same with a programming language of your choice)

Exercise 3.1.1.1 build a NPM executable

1. Create an Image which can run npm install. Create a package.json and copy it inside the image. Run this image and note if the dependencies are installed every time. Why?
2. Change the Image to install the dependencies in the Image itself.
3. Share the node_modules folder when running the container so that npm runs inside the container but the dependencies get installed outside.

Exercise 3.1.1.2 Dockerize a Node app

1. Take the source at https://github.com/steotia/sample-nodejs and dockerize it to run on port 3000.

Hint: You will need to take a base image with node, copy the source code, do an npm install, expose port 3000 and finally run it