Add support for php 8.1

This commit adds optional support for basing images on php 8.1 as
opposed to php 8.0.

internal/wisski/ingredient/barrel/stack.go

@@ -31,6 +31,8 @@ func (barrel *Barrel) Stack() component.StackWithResources {
 
 			"DATA_PATH":   filepath.Join(barrel.FilesystemBase, "data"),
 			"RUNTIME_DIR": barrel.Malt.Config.Paths.RuntimeDir(),
+
+			"BARREL_BASE_IMAGE": barrel.DockerBaseImage,
 		},
 
 		MakeDirs: []string{"data", ".composer"},