Various context fixups

[jedevc]

🐛 More context bug fixes 🎉

See commit messages for more details.

1. Fixes resolving the context for remote bake files over SSH:

   $ docker buildx bake git@github.com:docker/buildx.git
   [+] Building 3.8s (2/2) FINISHED                                                                                                                                 docker-container:container
    => CACHED [internal] load git source git@github.com:docker/buildx.git                                                                                                                 3.2s
    => ERROR [internal] load git source git@github.com:docker/buildx.git                                                                                                                  0.0s 
   ------                                                                                                                                                                                      
   > [internal] load git source git@github.com:docker/buildx.git:
   ------
   ERROR: failed to solve: failed to read dockerfile: failed to load cache key: unset ssh forward key default
   

   bake: forward default ssh agent for remote bake file #1711 only fixed this for --print, we actually do two loads of the same context: one for bake to read the file, and another for the build to read the contents of the Dockerfile.

2. Fixes resolving dockerfile URLs for the controller:

   $ docker buildx build -f https://raw.githubusercontent.com/docker/buildx/master/hack/dockerfiles/lint.Dockerfile .
   INFO: connecting to buildx server
   INFO: no buildx server found; launching...
   [+] Building 0.0s (0/0)                                                                                                                                                                     
   ERROR: failed to build: rpc error: code = Unknown desc = could not find /home/jedevc/Documents/Docker/buildx/https:/raw.githubusercontent.com/docker/buildx/master/hack/dockerfiles: stat /home/jedevc/Documents/Docker/buildx/https:/raw.githubusercontent.com/docker/buildx/master/hack/dockerfiles: no such file or directory
   

   build: fixup resolvePaths for remote context path #1701 only fixes this for the context path, I wasn't aware that the Dockerfile path is explicitly allowed to be an HTTP URL:

   buildx/build/build.go

   Lines 1314 to 1323 in 672eeed

   	if urlutil.IsURL(inp.DockerfilePath) {
   	dockerfileDir, err = createTempDockerfileFromURL(ctx, d, inp.DockerfilePath, pw)
   	if err != nil {
   	return nil, err
   	}
   	toRemove = append(toRemove, dockerfileDir)
   	dockerfileName = "Dockerfile"
   	target.FrontendAttrs["dockerfilekey"] = "dockerfile"
   	delete(target.FrontendInputs, "dockerfile")
   	}