Pull platform specific containers by default to save bandwidth (by 20x for some images)

The default behaviour results in pulling images sometimes >20x greater than what's needed.

Eg. attempting to pull the `python` image started an 8GB download:
```
container images pull python
⠦ [1/2] Fetching image 0% (15 of 112 blobs, 2.0 MB/7.92 GB, 810 KB/s) [9s]
```

Specifying my local platform reduces this substantially:
```
container images pull --platform linux/arm64/v8 python
⠇ [1/2] Fetching image 4% (2 of 10 blobs, 14.4/354.7 MB, 554 KB/s) [29s]
```

Pulling images for the local platform by default would:
* Save users time (eg pulling the multi-platform python above would take ~3 hours)
* Save users confusion (I wondered whether it was pulling the wrong image, knowing that it's usually far smaller)
* Save bandwidth for docker.io who could otherwise get hammered by people pulling for the wrong platforms
* Save the planet 🌏

This project looks great btw, very exciting, congratulations for the launch! 🚀