feat: Provide TARGETARCH, TARGETOS, TARGETPLATFORM to docker builds

[rfay]

The Issue

I happened to notice that we kept having to provide ARG TARGETPLATFORM to some Dockerfile recipes

How This PR Solves The Issue

Automatically provide TARGETARCH, TARGETOS, TARGETPLATFORM so people don't need that.

TODO

• Update docs showing these in build-time environment variables (https://ddev.readthedocs.io/en/stable/users/extend/customizing-images/#build-time-environment-variables)
• In docs, show use in examples
• Add these in a test

Manual Testing Instructions

I used this Dockerfile:

RUN printf "TARGETPLATFORM=${TARGETPLATFORM}\nTARGETARCH=${TARGETARCH}\nTARGETOS=${TARGETOS}\n" >/var/lib/targetplatform.txt

And then ran ddev exec at /var/lib/targetplatform.txt after ddev start

Automated Testing Overview

Related Issue Link(s)

Release/Deployment Notes

[github-actions]

Download the artifacts for this pull request:

• all-ddev-executables.zip
• ddev-macos-amd64.zip
• ddev-macos-arm64.zip
• ddev-linux-arm64.zip
• ddev-linux-amd64.zip
• ddev-windows-amd64-installer.zip

See Testing a PR.

[stasadev]

It looks great!

Please add these variables to https://ddev.readthedocs.io/en/stable/users/extend/customizing-images/#build-time-environment-variables

And maybe we need an example how use them.

[rfay]

Converted to draft. TODO includes:

• Docs
• test
• Example