(release/v1.2) build: Makefile to build Dgraph inside docker container.

contrib/docker-build/Makefile

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+install:
+	@docker-compose up
+	@sudo chown $(USER) ../../dgraph/dgraph
+	@mv ../../dgraph/dgraph $(GOPATH)/bin

contrib/docker-build/README.md

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+# Docker build script
+
+This directory contains a Makefile that can be used to build Dgraph inside the
+official Dgraph Docker container. This is useful for situations when the host
+system cannot be used to build a binary that will work with the container (for
+example, if the host system has a different version of glibc).
+
+## Usage
+
+Run `make install` in this directory. The script will ask you for your password
+in order to change ownership of the compiled binary. By default, files written
+by Docker will be owned by root. This script also takes care of moving the
+binary to $GOPATH/bin.

contrib/docker-build/build.sh

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+#!/bin/bash
+
+export DEBIAN_FRONTEND=noninteractive
+apt-get update
+apt-get install -y build-essential git golang
+cd /dgraph/dgraph
+make

contrib/docker-build/docker-compose.yml

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+version: "3.5"
+services:
+  build:
+    image: dgraph/dgraph:latest
+    container_name: build
+    working_dir: /data/build
+    labels:
+      cluster: test
+      service: build
+    volumes:
+      - type: bind
+        source: ../../
+        target: /dgraph
+    command: /dgraph/contrib/docker-build/build.sh