CI(deps): Update docker/dockerfile Docker tag to v1.7

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
docker/dockerfile	syntax	minor	1.6 -> 1.7

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[echoix]

The release notes are shown here: https://github.com/moby/buildkit/releases/tag/dockerfile%2F1.7.0, 5 items in the list

1 is variable expansion supporting substitutions and trimming, otherwise pretty much some fixes. Reading the diff is pretty much impossible since it is a repo with more than one project that is released from it, and there's over 800 commits from more than a year.

Since they now suggest that you'd just pin to dockerfile:1, and if we'd done this, we'd never see the change, I would just merge and observe it, since no docker builds were done here. It doesn't worry me.