Update dockerfile frontend to 1.2, buildkit support graduated from experimental

[absoludity]

Description of the change

Noticed while adding a Dockerfile that we were still referring to the experimental (unversioned) docker frontend even though the buildkit support (for which it was added) graduated into 1.2.

Benefits

Stay on a stable API.

Possible drawbacks

Let's see what CI says.

[antgamdia]

Interesting! Thanks for upgrading this!
Just a couple of doubts:

• I guess this is for dockerfiles using a builder image. In that case, should we add the # syntax... also to the pinniped-proxy image?
• Since it uses semver, do we really want to stick to the minor version, or should we replace it by just docker/dockerfile:1?

[absoludity]

Interesting! Thanks for upgrading this!
Just a couple of doubts:

* I guess this is for dockerfiles using a builder image. In that case, should we add the `# syntax...` also to the pinniped-proxy image?

The only reason it was added for the go images was to save re-downloading all the data when building. That said, it'd be useful for the pinniped-proxy image for the same reason, for sure.

* Since it [uses semver](https://docs.docker.com/engine/reference/builder/#official-releases), do we really want to stick to the minor version, or should we replace it by just `docker/dockerfile:1`?

It explicitly needs to be >= 1.2 . As long as it'll grab the latest by default, then yep, that should be fine.

[absoludity]

FWIW, that simplified the pinniped-proxy Dockerfile considerably, and improved the speed of the dev setup.