Slides on talk given on Docker Projects 4th Birthday Celebration event in Kathmandu. Talk was about Dockerfiles and good practices while writing them. If there are any mistakes in the slides, please kindly mention it in the comments such that I myself as well as others who view the slide can benefit from it.

1. Dockerfiles & Best Practices
#BDay4

2. Avash Mulmi
● Mozilla Representative for Nepal
● Volunteer, FOSS Nepal Community
● Docker Meetup Organizer

3. Agenda
● What are Dockerfiles?
● Usage
● Best Practices

4. Dockerfile(s)?
● Textfile, named Dockerfile
● Contains commands, in sequential order
● Used to build an image
● Have own format and instruction set

5. Dockerfile

6. Dockerfile(s)?

7. Building Images
● Change WORKDIR to Folder with Dockerfile (General Practice)
● ~# docker build .
● ~# docker build -t IMAGE_NAME:Tag .
● ~# docker build -t IMAGE_NAME:Tag -f Dockerfile /path/to/Dockerfile

8. Working with Built Image
● Check for built image:
 ~# docker images
● Saving built image:
 ~# docker save IMAGENAME > image.tar
● Deleting/Removing built image:
 ~# docker rmi IMAGENAME
 ~# docker rmi IMAGE_ID

9. Best Practices
● Ephemeral Containers
● Using .dockerignore file
● As lightweight as possible
● Minimize the number of layers
● One container, One purpose
● Use volumes
● Avoid using root user

10. Ephemeral Containers
● Containers should be ephemeral
● Literal meaning of Ephemeral → Short lived, brief, momentary
● Stopped, Destroyed and New one built with minimum set-up &
configurations
● Good and Complete Dockerfile can achieve this easily

11. .dockerignore
● Similar to .gitignore
● Generally Dockerfile is in an empty directory
● In some cases, we need to add files and directories to the
containers
● In such cases, use .dockerignore in those directories to skip files
that are not required at all

12. Lightweight
● Install only required packages
● It can reduce complexity, dependencies, file size and build times
● Some applications ‘might’ look important for future
● For eg: a text editor in a database image
● Make image as small as possible using light base images where
possible

13. Minimize Number of Layers
● Each RUN command creates a new layer
● Need to know figure out how to minimize the number of layers
● Example 1: update, upgrade and installation of packages can stay
in a single RUN command with multi-line arguments
● Example 2: download of a zip file, extraction and move to another
folder
● Benefits: docker pull and docker push are faster, reduces
complexity of an image etc.

14. Example 1
● RUN apt-get update
RUN apt-get install apache2 git mysql-server php5 vim
RUN apt-get clean
● RUN apt-get update
&& apt-get install apache2 git mysql-server php5 vim
&& apt-get clean

15. Example 2
● RUN curl http://192.168.100.5/file.tar.gz
RUN tar xvf file.tar.gz
RUN mv EXTRACTED_FILE /opt/
● RUN curl http://192.168.100.5/file.tar.gz | tar xvf -C /opt/

16. Example 3
● RUN apt-get update
&& apt-get install apache2 git mysql-server php5 vim
&& apt-get clean
RUN curl http://192.168.100.5/file.tar.gz | tar xvf -C /opt/
RUN cp /etc/someconfig /opt/someconfig
&& ln -s /usr/share/app1/default.cfg /opt/default.cfg
● RUN apt-get update
&& apt-get install apache2 git mysql-server php5 vim
&& apt-get clean
&& curl | tar xvf -C /opt/
&& cp /etc/someconfig /opt/someconfig
&& ln -s /usr/share/app1/default.cfg /opt/default.cfg
&& cd /opt/ && ./configure && make && make install
&& rm old_unnecessary_files

17. One Container, One Purpose
● Keep containers as clean and modular as possible
● Creating containers for different dependent applications and
linking them makes it easier to maintain & reuse those containers
● Eg: Instead of single container with webserver, database and
(php), three separate containers with webserver, database and
(php) in each is better.
● Why? → Maintenance & Reusablity

18. Use Volumes
● Do not store data in containers, as far as possible
● Use Volumes to store data
● Concept of ephemeral
● App container running v1.9 should be easily stopped, destroyed
and updated with container running v2.0 without losing any data

19. Avoid using `root` user
● By default docker runs containers with root user
● If possible use a normal user
● Depends on situation

20. Delete unnecessary files
● Reduces size of image and container
● Eg:
RUN curl https://192.168.5.100/file.tar.gz
&& tar xvf -C file.tar.gz /opt/
&& rm file.tar.gz

21. Use comments
● Comments can help users as well as yourself in the future.
● Dockerfile comments start with ‘#’
● Eg:
#Install required packages for the app
RUN apt-get install package1 package2 package3
# Modifying texts from default config
RUN sed -i ‘s/word1/word2/g’ config.cfg

22. ADD & COPY
● COPY only copies file from directory
● ADD allows source to be URL
● ADD also has functionality of `tar`, archiving
● Use COPY as far as possible as it is more transparent.
● ADD may add malicious files from unknown sources if used
improperly.

23. References
● Dockerfile references
https://docs.docker.com/engine/reference/builder/
● 10 things to avoid in docker containers
https://developers.redhat.com/blog/2016/02/24/10-things-to-avoid-in-d
● Docker best practices
https://github.com/FuriKuri/docker-best-practices

24. avasz@protonmail.com