c8d: Skip the test that checks Config.Image

It's not set when containerd is used as an image store and buildkit
never sets it either, so let's skip this test if snapshotters are used

Signed-off-by: Djordje Lukic <djordje.lukic@docker.com>

integration-cli/docker_cli_by_digest_test.go

@@ -17,6 +17,7 @@ import (
 	"github.com/opencontainers/go-digest"
 	"gotest.tools/v3/assert"
 	is "gotest.tools/v3/assert/cmp"
+	"gotest.tools/v3/skip"
 )
 
 var (
@@ -180,6 +181,7 @@ func (s *DockerRegistrySuite) TestRemoveImageByDigest(c *testing.T) {
 }
 
 func (s *DockerRegistrySuite) TestBuildByDigest(c *testing.T) {
+	skip.If(c, testEnv.UsingSnapshotter(), "Config.Image is not created with containerd, buildkit doesn't set it either")
 	digest, err := setupImage(c)
 	assert.NilError(c, err, "error setting up image")