duneci-build-docker-image: HTTP headers might be lower case
The new Gitlab installation has HTTP/2 enabled. curl makes use of this, but then returns header names in lower case. So we need to do case-insensitive matching in order to retrieve it.
The new Gitlab installation has HTTP/2 enabled. curl makes use of this, but then returns header names in lower case. So we need to do case-insensitive matching in order to retrieve it.