Add Documentation for --opt for frontends (i.e. Dockerfile.v0)

[sluetze]

trafficstars

Hi, at the moment of writing there is no Documentation for the options (--opt) for the Dockerfile frontend. This makes it necessary to dive into the sourcecode for understanding how to set a label with buildctl.

The only place to get an idea which options are supported/possible is https://github.com/moby/buildkit/blob/master/frontend/dockerfile/builder/build.go and https://github.com/moby/buildkit/blob/master/frontend/dockerfile/dockerfile2llb/convert.go

If enhancements in documentation are welcome, I could try to contribute. I am also not sure, were the best place for this would be.

Internal in https://github.com/moby/buildkit/tree/master/frontend/dockerfile/docs, as this is specific to the frontend, or in the README.md as there is a small example with build-args existing. It feels more user-friendly to enhance the README.

-- sluetze

[arcosx]

Hello, @sluetze I have written some documents about opts inside this PR https://github.com/moby/buildkit/pull/3068 , please help to see if it is enough and if it can help you, welcome to make some comments inside the PR.