Dockerfile: update to Fedora 40

[aaradhak]

coreos/fedora-coreos-tracker#1674

[aaradhak]

@jlebon I see that the release notes are already updated to Fedora 40 in this case - https://github.com/coreos/butane/blob/main/docs/release-notes.md

[jlebon]

@jlebon I see that the release notes are already updated to Fedora 40 in this case - main/docs/release-notes.md

Not quite. That release note entry refers to the butane binary. The container image itself being based on Fedora 40 is a separate change. Though it doesn't look like historically we've release noted it.

That said, this is missing another instance in the same file.

[prestist]

@aaradhak after you have updated both From's you can then add a label for "skip notes" as we dont generally doc the update of this.