Fix dockerfile linting issue with using different casing

See https://docs.docker.com/reference/build-checks/from-as-casing/ for
details. This change fixes an warning wich is shown in every PR.

.docker/prod.Dockerfile

@@ -1,7 +1,7 @@
 ARG VERSION=stable
 ARG DEBIAN_FRONTEND=noninteractive
 
-FROM debian:stable-slim as builder
+FROM debian:stable-slim AS builder
 
 ENV NODE_VERSION=node_18.x
 ENV NODE_KEYRING=/usr/share/keyrings/nodesource.gpg