When building in version 3.2.1 "repository name must be lowercase"

Thank you for the hint, but the repository names seems to be in lower case:
docker-repo.mhkservice.net/macrocom/mariadb:v10.5.8.6
as it works in v3.1.0 there could be a problem in v3.2.1?
Is there a way to see the actually called repository name (some log file)?
Maybe some “sub”-dockerfiles in repo?

Im using a Make-file, docker-compose.yml, the Dockerfile and .env for the call:

.env:
PRIVATE_DOCKER_REPO_URL=docker-repo.mhkservice.net/macrocom
MYSQL_DOCKERIMAGE=mariadb:v10.5.8.6

docker-composer.yml:
mariadb-service:
build:
context: ./mariadb
target: DEV
args:
- DOCKER_REPO_URL={PRIVATE_DOCKER_REPO_URL} - DOCKERIMAGE={MYSQL_DOCKERIMAGE}
env_file:
- .env

Dockerfile:
FROM $DOCKER_REPO_URL/$DOCKERIMAGE as PROD