conformance tests: archive the context directory as 0:0

When uploading a context directory to dockerd, generate the archive
ourselves so that we can force the ownerships in it to 0:0, which
BuildKit seems to expect clients to do.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

tests/conformance/conformance_test.go

@@ -614,12 +614,18 @@ func buildUsingDocker(ctx context.Context, t *testing.T, client *docker.Client,
 		pullImageIfMissing(t, client, stageBase)
 	}
 
+	excludes, err := imagebuilder.ParseDockerignore(contextDir)
+	require.NoErrorf(t, err, "parsing ignores file in %q", contextDir)
+	inputReader, inputCloser, err := dockerclient.NewClientExecutor(client).Archive(true, contextDir, ".", false, excludes)
+	require.NoErrorf(t, err, "archiving context directory %q", contextDir)
+	defer inputCloser.Close()
+
 	// set up build options
 	output := &bytes.Buffer{}
 	options := docker.BuildImageOptions{
 		Context:             ctx,
-		ContextDir:          contextDir,
 		Dockerfile:          dockerfileRelativePath,
+		InputStream:         inputReader,
 		OutputStream:        output,
 		Name:                dockerImage,
 		NoCache:             true,