bake: allow dot in target names for compose

This is a hotfix for v0.8 to unblock release and
restore backward compatibility. More proper fix
coming later.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

bake/compose_test.go

@@ -330,6 +330,10 @@ func TestServiceName(t *testing.T) {
 		},
 		{
 			svc:     "a.b",
+			wantErr: false,
+		},
+		{
+			svc:     "a?b",
 			wantErr: true,
 		},
 		{