Add simple test to image before uploading

[pheki]

• Pushing only (with the image name changed to my fork): run, diff
• Whole run: https://github.com/pheki/vita-rust-docker/actions/runs/7536344986

This would help us:

• Not push images with broken compiler / std versions
• Realize sooner when the compiler / std breaks (although for this one we also probably want to build all examples)

[nikarh]

Makes a lot of sense, thanks!
Approved, merge it when you feel like it :)

[nikarh]

Also, if you know the last good nightly, please rerun the workflow (there's a manual trigger where you can enter the version to use), so that image:latest is the working one :)

[pheki]

if you know the last good nightly, please rerun the workflow, so that image:latest is the working one

Oh right, done! For reference it's nightly-2024-01-04