Improve Docker instructions in docs/usage.md

[TimWolla]

This aims to strike a balance between “best practice” and “being easy to understand”. It uses RUN --mount to avoid including PIE in the resulting image and takes care to uninstall unzip as well, but does not provide “extension mechanisms” to install per-extension build or runtime libraries.

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The “long example” is now self-contained including a demo CMD:

$ docker build -t test .
[+] Building 0.2s (8/8) FINISHED                                                                                                                                                                                                                                                                                                                                              docker:rootless
 => [internal] load build definition from Dockerfile                                                                                                                                                                                                                                                                                                                                     0.0s
 => => transferring dockerfile: 641B                                                                                                                                                                                                                                                                                                                                                     0.0s
 => [internal] load metadata for ghcr.io/php/pie:bin                                                                                                                                                                                                                                                                                                                                     0.2s
 => [internal] load metadata for docker.io/library/php:8.4-cli                                                                                                                                                                                                                                                                                                                           0.0s
 => [internal] load .dockerignore                                                                                                                                                                                                                                                                                                                                                        0.0s
 => => transferring context: 2B                                                                                                                                                                                                                                                                                                                                                          0.0s
 => FROM ghcr.io/php/pie:bin@sha256:24df295bb3dbf6cbcb42f49a14eaef043b99ccaa17f8412f3c04eb330e6da12d                                                                                                                                                                                                                                                                                     0.0s
 => [stage-0 1/2] FROM docker.io/library/php:8.4-cli                                                                                                                                                                                                                                                                                                                                     0.0s
 => CACHED [stage-0 2/2] RUN --mount=type=bind,from=ghcr.io/php/pie:bin,source=/pie,target=/usr/local/bin/pie     export DEBIAN_FRONTEND="noninteractive";     set -eux;     apt-get update;     apt-get install -y --no-install-recommends unzip;     pie install asgrim/example-pie-extension;     apt-get purge -y --auto-remove -o APT::AutoRemove::RecommendsImportant=false unzip  0.0s
 => exporting to image                                                                                                                                                                                                                                                                                                                                                                   0.0s
 => => exporting layers                                                                                                                                                                                                                                                                                                                                                                  0.0s
 => => writing image sha256:d8b2494ffd6f830fcb6fde1ca9cd511fac7c6d52f5b9dae92f6fe26fd8e37306                                                                                                                                                                                                                                                                                             0.0s
 => => naming to docker.io/library/test                                                                                                                                                                                                                                                                                                                                                  0.0s
$ docker run -it --rm test
Hello, world!

It comes with just 1.4 MB of additional storage requirements (not the full 11 MB of PIE):

$ docker history test 
IMAGE          CREATED         CREATED BY                                      SIZE      COMMENT
d8b2494ffd6f   6 minutes ago   CMD ["php" "-r" "example_pie_extension_test(…   0B        buildkit.dockerfile.v0
<missing>      6 minutes ago   RUN /bin/sh -c export DEBIAN_FRONTEND="nonin…   1.41MB    buildkit.dockerfile.v0

PR submitter checklist

• I have read CONTRIBUTING.md
• I discussed this <bug|feature> with the maintainers in #<issue_number> (complete as appropriate)
• I have added appropriate tests
• I confirm that I have the right to submit this under the project's open source licence