Use = to make ENV happy

[daveisfera]

https://docs.docker.com/engine/deprecated/#dockerfile-legacy-env-name-value-syntax

[tianon]

Thanks for the contribution! Unfortunately, this isn't something we're likely to accept: moby/buildkit#5130 😇

[daveisfera]

Definitely appreciate that there appears to be an internal war on this one, but can this be merged? It's a minor change that I imagine is a lot easier than removing that deprecation warning from software that's been deployed around the world for several years now.
In the end, it's not really an impact on me or any users, but I grabbed this image to build my own free-threading variant of 3.14 and would have been nice to not see warnings and wonder if they impacted the build (i.e. the wasted human time on this matter is the real big deal on issues like this one)