build(standalone): Add DGRAPH_VERSION to build different Dgraph versions for standalone image. (#6082) (#6238)

The standalone images build from the same tags as the `dgraph/dgraph` image.

To build and push the v20.03.4 and latest `dgraph/standalone` images:

    # Build v20.03.4
    make DGRAPH_VERSION=v20.03.4
    make DGRAPH_VERSION=v20.03.4 push

    # Build latest. DGRAPH_VERSION defaults to latest
    make
    make push

Changes
* Add `DOCKER_IMAGE` and `DGRAPH_VERSION` arguments to build standalone Docker image.
* Add ./hooks folder for Docker Hub auto-builds with `--build-arg`s (following [Custom build phase hooks](https://docs.docker.com/docker-hub/builds/advanced/#custom-build-phase-hooks) docs).

Author: danielmai

contrib/standalone/Dockerfile

@@ -1,4 +1,5 @@
-FROM dgraph/dgraph:latest
+ARG DGRAPH_VERSION=latest
+FROM dgraph/dgraph:${DGRAPH_VERSION}
 LABEL MAINTAINER="Dgraph Labs <[email protected]>"
 
 # Ratel port

contrib/standalone/Dockerfile.master

@@ -1,3 +1,38 @@
-.PHONY: build
+# Build dgraph/standalone image
+# Usage:
+#
+# For latest tag:
+#     Build, based on dgraph/dgraph:latest:
+#       make
+#     Push:
+#       make push
+#
+# For $TAG tag, set DGRAPH_VERSION:
+#     Build, based on dgraph/dgraph:latest:
+#       make DGRAPH_VERSION=$TAG
+#     Push:
+#       make DGRAPH_VERSION=$TAG push
+#
+# Examples
+#     Build and push latest:
+#       make
+#       make push
+#
+#     Build and push master:
+#       make DGRAPH_VERSION=master
+#       make DGRAPH_VERSION=master push
+#
+.PHONY: all build push version
+
+export DGRAPH_VERSION ?= latest
+
+all: build version
+
 build:
-	docker build -t dgraph/standalone:latest .
+	./hooks/build
+
+push:
+	docker push dgraph/standalone:$(DGRAPH_VERSION)
+
+version:
+	docker run dgraph/standalone:$(DGRAPH_VERSION) dgraph version

contrib/standalone/Makefile

@@ -0,0 +1,3 @@
+#!/bin/bash
+# Used for Makefile or Docker Hub builds
+docker build -t dgraph/standalone:${DGRAPH_VERSION} --build-arg DGRAPH_VERSION=${DGRAPH_VERSION} .