Dockerfile Syntax

Ref: https://learn.cantrill.io/courses/docker-fundamentals/lectures/44660838 =OR= https://www.youtube.com/watch?v=cB4VKZI2sVw

DEMO: https://learn.cantrill.io/courses/docker-fundamentals/lectures/44660840 =OR= https://www.youtube.com/watch?v=snXchrxRoPc → GitHub: https://github.com/acantril/docker-fundamentals/blob/main/build-a-simple-containerized-application/build-a-simple-containerized-application.md

Summary
Contents:

Building images with Dockerfile

🔧 A Dockerfile contains a set of instructions/steps/commands to create a Docker image
Command: docker build -t <TAG> <Dockerfile_path>
- Uses Dockerfile found in <Dockerfile_path> to build a new image
- <Dockerfile_path> can be e.g. current working directory, i.e. .
- New image gets tagged with <TAG>
💡 When building images, consider how general purpose the image will be
- The more layers you add, the more general purpose the image is…
- …but there should be a good reason to add layers
  - containers are supposed to be resource efficient
  - layers add build and run overhead

Dockerfile basic keywords

FROM - set the base image for a build
- e.g. Alpine → lightweight (~5MB) Linux distro, ideal for Docker usage
LABEL - add metadata to an image
- e.g. description, maintainer…
❗ Keywords that add a new layer
- RUN - run commands in a new layer
  - e.g. install and configure new SW
- COPY - copy files/directories from source directory in client machine to destination directory inside new image layer
- ADD - like COPY, but source can be a remote URL and additional extraction can be done
  - e.g. adding application or web files from internet repository
Image execution keywords
- CMD - set default executable of a container & arguments
  - e.g. run a web server
  - can be overridden using parameters in docker run command
- ENTRYPOINT - same as CMD, but can't be overridden
  - ideal for single purpose containers
- ❗ CMD and ENTRYPOINT can be used in the same Dockerfile
  - In this case, CMD indicates the default parameters for ENTRYPOINT
Find more commands in official documentation: https://docs.docker.com/reference/dockerfile/

Dockerfile Example

FROM nginx:latest
LABEL [maintainer="[email protected]](mailto:maintainer=%[email protected])"
COPY 2048 /usr/share/nginx/html
EXPOSE 80
CMD ["nginx", "-g", "daemon off;"]

Use latest version of the nginx image
- includes a configured installation of NGINX
- https://hub.docker.com/_/nginx
Set metadata maintainer