fix docs on adding PHP 8.1

Author: fontebasso

Dockerfile

@@ -5,8 +5,8 @@ FROM ${NAME_IMAGE_BASE}:${NAME_IMAGE_TAG}
 
 ARG BUILD_ID="unknown"
 ARG COMMIT_ID="unknown"
-ARG VERSION_OS='3.13'
-ARG VERSION_PHP='8.0'
+ARG VERSION_OS='3.15'
+ARG VERSION_PHP='8.1'
 
 LABEL \
     ALPINE="$VERSION_OS" \