cli: Fix docker sftp volume mapping (#21547)

The previous code was not mapping the volumes correctly. It seemed to be
assuming it was mapping over the target paths rather than the
"src:target" strings.

Also it seems we need to put everything under /home/wordpress, otherwise
the symlinks fail.

tools/cli/helpers/docker-config.js

@@ -177,9 +177,7 @@ const setMappings = ( argv, config ) => {
 
 	if ( argv.type === 'dev' ) {
 		mappingsCompose.services.sftp = {
-			volumes: volumesMapping.map( vol =>
-				vol.startsWith( '/var/www/html' ) ? '/home/wordpress' + vol : vol
-			),
+			volumes: volumesMapping.map( vol => vol.replace( ':', ':/home/wordpress' ) ),
 		};
 	}