fix: user proper keywords casing in Dockerfile

Fix warning when building the epubcheck Docker image[1]
(Docker server version: 27.0.3, API version: 1.46):

> WARN: FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 2)

[1]: https://docs.docker.com/reference/build-checks/from-as-casing/

Dockerfile

@@ -1,5 +1,5 @@
 # build the epubcheck.jar file
-FROM maven:slim as builder
+FROM maven:slim AS builder
 
 WORKDIR /app
 COPY . .