Buildkit: Add documentation (/support?) changing dockerfile name with dockerfile frontend

[TeNNoX]

I fail to use a differently-named dockerfile (other.dockerfile), I tried different flags from reading the code, but all attempts failed:

Command:

buildctl-daemonless.sh build --progress=plain \
      --frontend=dockerfile.v0 ...

--local dockerfile=. --local filename=other.dockerfile
-> error: other.dockerfile not a directory

--local dockerfile=other.dockerfile
-> error: other.dockerfile not a directory

--local dockerfile=. --local dockerfilekey=other.dockerfile
-> error: other.dockerfile not a directory

--local dockerfile=. --local defaultDockerfileName=other.dockerfile
-> error: other.dockerfile not a directory

Using moby/buildkit:latest - buildctl github.com/moby/buildkit v0.6.3 928f3b480d7460aacb401f68610058ffdb549aca

[AkihiroSuda]

--opt filename=other.dockerfile

[redbaron]

For the reference, list of opts can be found here: https://github.com/moby/buildkit/blob/48f4a9b00ea7b1c7cac22cee76156cb13790406b/frontend/dockerfile/builder/build.go#L30-L53