Quote variables, actually scope in our arguments

Dockerfile

@@ -31,11 +31,15 @@ FROM alpine:latest AS final
 
 # Use only a subset of arguments
 ARG JRE_VERSION
+ARG UID
+ARG USER
+ARG GID
+ARG GROUP
 
 # Build the thing up
 RUN apk add bash curl findutils rsync screen "${JRE_VERSION}"
-RUN addgroup -g $GID $USER && \
-	adduser -h /minecraft -s /bin/sh -G $GROUP -D -H -u $UID
+RUN addgroup -g "${GID}" "${USER}" && \
+	adduser -h /minecraft -s /bin/sh -G "${GROUP}" -D -H -u "${UID}"
 WORKDIR /minecraft
 COPY --from=build /minecraft .
 USER $USER