Fix a dockerfile linting warning

This warning is raised in the GitHub Action CI on every PR. FROM and AS
should use the same folding.

.docker/prod.Dockerfile

@@ -1,6 +1,6 @@
 ARG GVM_LIBS_VERSION=oldstable
 
-FROM greenbone/gvm-libs:${GVM_LIBS_VERSION} as builder
+FROM greenbone/gvm-libs:${GVM_LIBS_VERSION} AS builder
 
 # This will make apt-get install without question
 ARG DEBIAN_FRONTEND=noninteractive