Add a section leading to multi-stage builds

slides/intro/Multi_Stage_Builds.md

@@ -1,4 +1,4 @@
-# Multi-stage builds
+# Reducing image size
 
 * In the previous example, our final image contained:
 
@@ -14,7 +14,196 @@
 
 ---
 
-## Multi-stage builds principles
+## Can't we remove superfluous files with `RUN`?
+
+What happens if we do one of the following commands?
+
+- `RUN rm -rf ...`
+
+- `RUN apt-get remove ...`
+
+- `RUN make clean ...`
+
+--
+
+This adds a layer which removes a bunch of files.
+
+But the previous layers (which added the files) still exist.
+
+---
+
+## Removing files with an extra layer
+
+When downloading an image, all the layers must be downloaded.
+
+| Dockerfile instruction | Layer size | Image size |
+| ---------------------- | ---------- | ---------- |
+| `FROM ubuntu` | Size of base image | Size of base image |
+| `...` | ... | Sum of this layer <br/>+ all previous ones |
+| `RUN apt-get install somepackage` | Size of files added <br/>(e.g. a few MB) | Sum of this layer <br/>+ all previous ones |
+| `...` | ... | Sum of this layer <br/>+ all previous ones |
+| `RUN apt-get remove somepackage` | Almost zero <br/>(just metadata) | Same as previous one |
+
+Therefore, `RUN rm` does not reduce the size of the image or free up disk space.
+
+---
+
+## Removing unnecessary files
+
+Various techniques are available to obtain smaller images:
+
+- collapsing layers,
+
+- adding binaries that are built outside of the Dockerfile,
+
+- squashing the final image,
+
+- multi-stage builds.
+
+Let's review them quickly.
+
+---
+
+## Collapsing layers
+
+You will frequently see Dockerfiles like this:
+
+```dockerfile
+FROM ubuntu
+RUN apt-get update && apt-get install xxx && ... && apt-get remove xxx && ...
+```
+
+Or the (more readable) variant:
+
+```dockerfile
+FROM ubuntu
+RUN apt-get update \
+ && apt-get install xxx \
+ && ... \
+ && apt-get remove xxx \
+ && ...
+```
+
+This `RUN` command gives us a single layer.
+
+The files that are added, then removed in the same layer, do not grow the layer size.
+
+---
+
+## Collapsing layers: pros and cons
+
+Pros:
+
+- works on all versions of Docker
+
+- doesn't require extra tools
+
+Cons:
+
+- not very readable
+
+- some unnecessary files might still remain if the cleanup is not torough
+
+- that layer is expensive (slow to build)
+
+---
+
+## Building binaries outside of the Dockerfile
+
+This results in a Dockerfile looking like this:
+
+```dockerfile
+FROM ubuntu
+COPY xxx /usr/local/bin
+```
+
+Of course, this implies that the file `xxx` exists in the build context.
+
+That file has to exist before you can run `docker build`.
+
+For instance, it can:
+
+- exist in the code repository,
+- be created by another tool (script, Makefile...),
+- be created by another container image and extracted from the image.
+
+See for instance the [busybox official image](https://github.com/docker-library/busybox/blob/fe634680e32659aaf0ee0594805f74f332619a90/musl/Dockerfile) or this [older busybox image](https://github.com/jpetazzo/docker-busybox).
+
+---
+
+## Building binaries outside: pros and cons
+
+Pros:
+
+- final image can be very small
+
+Cons:
+
+- requires an extra build tool
+
+- we're back in dependency hell and "works on my machine"
+
+Cons, if binary is added to code repository:
+
+- breaks portability across different platforms
+
+- grows repository size a lot if the binary is updated frequently
+
+---
+
+## Squashing the final image
+
+The idea is to transform the final image into a single-layer image.
+
+This can be done in (at least) two ways.
+
+- Activate experimental features and squash the final image:
+  ```bash
+  docker image build --squash ...
+  ```
+
+- Export/import the final image.
+  ```bash
+  docker build -t temp-image .
+  docker run --entrypoint true --name temp-container temp-image
+  docker export temp-container | docker import - final-image
+  docker rm temp-container
+  docker rmi temp-image
+  ```
+
+---
+
+## Squashing the image: pros and cons
+
+Pros:
+
+- single-layer images are smaller and faster to download
+
+- removed files no longer take up storage and network resources
+
+Cons:
+
+- we still need to actively remove unnecessary files
+
+- squash operation can take a lot of time (on big images)
+
+- squash operation does not benefit from cache
+  <br/>
+  (even if we change just a tiny file, the whole image needs to be re-squashed)
+
+---
+
+## Multi-stage builds
+
+Multi-stage builds allow us to have multiple *stages*.
+
+Each stage is a separate image, and can copy files from previous stages.
+
+We're going to see how they work in more detail.
+
+---
+
+# Multi-stage builds
 
 * At any point in our `Dockerfile`, we can add a new `FROM` line.