Ensure BaseName of Stage is not blank

Related: https://github.com/moby/moby/issues/37325

Signed-off-by: Yuichiro Kaneko <spiketeika@gmail.com>

frontend/dockerfile/dockerfile2llb/convert.go

@@ -85,6 +85,9 @@ func Dockerfile2LLB(ctx context.Context, dt []byte, opt ConvertOpt) (*llb.State,
 		if err != nil {
 			return nil, nil, err
 		}
+		if name == "" {
+			return nil, nil, errors.Errorf("base name (%s) should not be blank", st.BaseName)
+		}
 		st.BaseName = name
 
 		ds := &dispatchState{

frontend/dockerfile/dockerfile2llb/convert_test.go

@@ -41,4 +41,12 @@ COPY --from=0 f2 /
 		Target: "nosuch",
 	})
 	assert.Error(t, err)
+
+	df = `FROM "" AS foo`
+	_, _, err = Dockerfile2LLB(appcontext.Context(), []byte(df), ConvertOpt{})
+	assert.Error(t, err)
+
+	df = `FROM ${BLANK} AS foo`
+	_, _, err = Dockerfile2LLB(appcontext.Context(), []byte(df), ConvertOpt{})
+	assert.Error(t, err)
 }