chore: use multistage dockerfile correctly from compose

docker/Dockerfile.application

@@ -9,7 +9,7 @@ WORKDIR $BUILD
 RUN --mount=type=cache,target=$MAVEN_HOME/.m2,uid=$BUILD_UID \
 	mvn -e -Duser.home=$MAVEN_HOME package
 
-FROM openjdk:7u211-jre-alpine
+FROM openjdk:7u211-jre-alpine AS backend
 RUN <<EOF
 	set -e
 	mkdir -p /app

docker/compose.yml

@@ -5,7 +5,8 @@ services:
   backend:
     build:
       context: ..
-      dockerfile: docker/Dockerfile.backend
+      dockerfile: docker/Dockerfile.application
+      target: backend
     restart: always
     environment:
       LW_CONFIGURE: 1