cli/compose/types: UlimitsConfig.MarshalYAML() fix recursion

When marshaling the type with `gopkg.in/yaml.v3`, unmarshaling would
recursively call the type's `MarshalYAML()` function, which ultimately
resulted in a crash:

    runtime: goroutine stack exceeds 1000000000-byte limit
    runtime: sp=0x140202e0430 stack=[0x140202e0000, 0x140402e0000]
    fatal error: stack overflow

This applies a similar fix as was implemented in e7788d6f9a
for the `MarshalJSON()` implementation. An alternative would be to use
a type alias (to remove the `MarshalYAML()`), but keeping it simple.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/compose/types/types.go

@@ -441,7 +441,8 @@ func (u *UlimitsConfig) MarshalYAML() (interface{}, error) {
 	if u.Single != 0 {
 		return u.Single, nil
 	}
-	return u, nil
+	// Return as a value to avoid re-entering this method and use the default implementation
+	return *u, nil
 }
 
 // MarshalJSON makes UlimitsConfig implement json.Marshaller