Dockerfile

Dockerfiles are part process workflow and part art

Commands

• Each of the commands mentioned below is an actual layer

• The order of the mentioned commands is important as it works top->down

FROM

• package managers like apt and yum are one of the reasons to build containers FROM Debian, Ubuntu, Fedora or CentOS

ENV - Environment Variables

• One reason they were chosen as preferred way to inject key/value is they work everywhere, on every OS and config

RUN

• Executing shell commands inside the container as it is building it

  • Install software from the package repository

  • Unziping

  • Create/Move/Delete some folders/files inside the container itself

  • Run shell scripts

  • Add additional repositories to package manager sources list

  • etc.

• Access to all the commands and binaries that have been installed with the OS release

• Add commands with ampersand '&&' sign to execute these commands as a one layer

  • When placing command on next line use the backslash '\'

LOGGING SETUP

• Everything has to be sent to stdout and stderr

  • We achieve this by creating symlink from log file to device file

# Example
RUN ln -sf /dev/stdout /var/log/nginx/access.log \
    && ln -sf /dev/stderr /var/log/nginx/error.log

EXPOSE

• Expose ports on the docker virtual network

• To open/forward ports to host -p or -P have to be used

CMD

• REQUIRED

• Final command that will be run everytime we launch new container from the image or everytime we restart/start container

• Only one CMD is allowed

  • If there are multiple, last one wins

Usage

• Build Image from Dockerfile in current directory

docker image build -t name .