feat: Update build.sbt and Dockerfile for improved GitHub credentials handling

Dockerfile

@@ -1,10 +1,6 @@
 # === Stage 1: Build the Play application ===
 FROM --platform=$BUILDPLATFORM sbtscala/scala-sbt:eclipse-temurin-alpine-22_36_1.10.3_3.5.1 AS builder
 
-ARG GITHUB_USER
-ARG GITHUB_TOKEN
-
-
 WORKDIR /app
 
 # Install Node.js and Less CSS preprocessor

build.sbt

@@ -40,8 +40,8 @@ lazy val knockoutwhistweb = project.in(file("knockoutwhistweb"))
     credentials += Credentials(
       "GitHub Package Registry",
       "maven.pkg.github.com",
-      sys.env("GITHUB_USER"),
-      sys.env("GITHUB_TOKEN")
+      sys.env.getOrElse("GITHUB_USER", sys.error("GITHUB_USER not set")),
+      sys.env.getOrElse("GITHUB_TOKEN", sys.error("GITHUB_TOKEN not set"))
     ),
 
     commonSettings,