chore: build the backend container

docker/Dockerfile.backend

@@ -0,0 +1,24 @@
+FROM maven:3-jdk-7 AS build
+ENV BUILD_UID=1000
+ENV BUILD=/src
+ENV MAVEN_HOME=/var/maven
+ENV MAVEN_CONFIG=/var/maven/.m2
+RUN mkdir -p $MAVEN_HOME && chown $BUILD_UID $MAVEN_HOME
+RUN mkdir -p $BUILD
+ADD .. $BUILD
+RUN chown -R $BUILD_UID $BUILD
+USER $BUILD_UID
+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
+RUN <<EOF
+	set -e
+	mkdir -p /app
+	chown 1000 /app
+EOF
+COPY --from=build /src/legacyworlds-server/legacyworlds-server-main/target/*.jar /app/server.jar
+COPY --from=build /src/legacyworlds-server/legacyworlds-server-main/target/lib /app/lib
+ADD docker/backend-entrypoint.sh /app/entrypoint.sh
+ENTRYPOINT ["/app/entrypoint.sh"]
+CMD ["server"]