Fix sub-folder baseURL handling for Page resources

I.e. images etc.

Fixes #4228

helpers/pathspec.go

@@ -26,6 +26,10 @@ import (
 type PathSpec struct {
 	BaseURL
 
+	// If the baseURL contains a base path, e.g. https://example.com/docs, then "/docs" will be the BasePath.
+	// This will not be set if canonifyURLs is enabled.
+	BasePath string
+
 	disablePathToLower bool
 	removePathAccents  bool
 	uglyURLs           bool
@@ -124,6 +128,13 @@ func NewPathSpec(fs *hugofs.Fs, cfg config.Provider) (*PathSpec, error) {
 		ProcessingStats:                NewProcessingStats(lang),
 	}
 
+	if !ps.canonifyURLs {
+		basePath := ps.BaseURL.url.Path
+		if basePath != "" && basePath != "/" {
+			ps.BasePath = basePath
+		}
+	}
+
 	publishDir := ps.AbsPathify(cfg.GetString("publishDir")) + FilePathSeparator
 	// If root, remove the second '/'
 	if publishDir == "//" {

helpers/url.go

@@ -319,12 +319,11 @@ func AddContextRoot(baseURL, relativePath string) string {
 // If canonifyURLs is set, we will globally prepend the absURL with any sub-folder,
 // so avoid doing anything here to avoid getting double paths.
 func (p *PathSpec) PrependBasePath(rel string) string {
-	basePath := p.BaseURL.url.Path
-	if !p.canonifyURLs && basePath != "" && basePath != "/" {
+	if p.BasePath != "" {
 		rel = filepath.ToSlash(rel)
 		// Need to prepend any path from the baseURL
 		hadSlash := strings.HasSuffix(rel, "/")
-		rel = path.Join(basePath, rel)
+		rel = path.Join(p.BasePath, rel)
 		if hadSlash {
 			rel += "/"
 		}