This document discusses multi-stage Docker images. It begins by explaining the problems that can occur from layers and image size when building images. It then defines that multi-stage images allow building an image in stages, where files can be copied between stages to create smaller final images. The document demonstrates how to build a multi-stage image using FROM to start stages, COPY to share files between stages, and naming stages for reference.

1. Multi Stage Docker
For fun and profit

2. Three Questions to Answer

3. Three Questions to Answer
1. What are multi-stage docker images?

4. Three Questions to Answer
1. What are multi-stage docker images?
2. Why would I want to use a multi-stage docker image?

5. Three Questions to Answer
1. What are multi-stage docker images?
2. Why would I want to use a multi-stage docker image?
3. How do I use a multi-stage docker image?

6. Three Questions to Answer
1. What are multi-stage docker images?
2. Why would I want to use a multi-stage docker image?
3. How do I use a multi-stage docker image?
But… I’m not going to answer them in that order

7. Why use a multi stage image?

8. Why use a multi stage image?
We had a Single Page App…

9. Why use a multi stage image?
We had a Single Page App…
We wanted to pre-render it to static files…

10. Why use a multi stage image?
We had a Single Page App…
We wanted to pre-render it to static files…
This used a headless chrome install…

11. Why use a multi stage image?
We had a Single Page App…
We wanted to pre-render it to static files…
This used a headless chrome install…
Also we wanted to serve the static files with a Perl6 Cro app…

12. Why use a multi stage image?
We had a Single Page App…
We wanted to pre-render it to static files…
This used a headless chrome install…
Also we wanted to serve the static files with a Perl6 Cro app...
Which caused problems because of Docker layers.

13. Layers and problems with them
Docker files never get smaller

14. Layers and problems with them
Docker files never get smaller
Each command in a Docker file creates a new layer.

15. Layers and problems with them
Docker files never get smaller
Each command in a Docker file creates a new layer.
Once a layer is added the image is expanded to that size.

16. Layers and problems with them
Docker files never get smaller
Each command in a Docker file creates a new layer.
Once a layer is added the image is expanded to that size.
Deleting anything in a later layer will remove the data but not shrink the layer.

17. Layers and problems with them
Docker files never get smaller
Each command in a Docker file creates a new layer.
Once a layer is added the image is expanded to that size.
Deleting anything in a later layer will remove the data but not shrink the layer.
Generally this is good and allows for caching and building up complex images.

18. Layers and problems with them
Docker files never get smaller
Each command in a Docker file creates a new layer.
Once a layer is added the image is expanded to that size.
Deleting anything in a later layer will remove the data but not shrink the layer.
Generally this is good and allows for caching and building up complex images.
But it can be a pain if you want a small image but need something big during creation.

19. What is a multi stage image?
Various solutions available generally involving
● Building an image with a mounted volume
● Building a second image and copying the data that was written to the volume
But these are brittle and a pain to maintain.
What would be awesome is to have some way of doing something like that in a single
Dockerfile.
Guess what? That’s what a multi stage image is.

20. How do I use a multi stage image?

21. How do I use a multi stage image?
FROM image1 Initial Starting Image

22. How do I use a multi stage image?
FROM image1
RUN job_that_creates_file
Initial Starting Image
Command to
generate data

23. How do I use a multi stage image?
FROM image1
RUN job_that_creates_file
FROM image2
Initial Starting Image
Command to
generate data
Second Stage Image

24. How do I use a multi stage image?
FROM image1
RUN job_that_creates_file
FROM image2
COPY --from=0 /path/to/file ./
Initial Starting Image
Command to
generate data
Second Stage Image
Copy Data from
Initial Image

25. Named Stages
FROM image1
RUN job_that_creates_file
FROM image2
COPY --from=0 /path/to/file ./

26. Named Stages
FROM image1 AS builder
RUN job_that_creates_file
FROM image2
COPY --from=builder /path/to/file ./
Give the initial image
a name
Stays the same
Use name in the
copy command

27. Further notes
You can have more than two stages in a build if you need
You can copy from a prebuilt image
EG: copy from=nginx:latest /etc/nginx/nginx.conf /nginx.conf
When building an image you can specify the stage to stop building at with --target
Full documentation is available on the Docker site :
https://docs.docker.com/develop/develop-images/multistage-build/

28. About Me
I’m Simon Proctor
Tech Lead at Zoopla working mostly in Devops and Backend stuff
We’re hiring
https://careers.zoopla.co.uk/

29. Questions?

30. Thanks to our sponsors