Docker improvements (#571)

- Copy the current checkout to the Dockerfile.
- Add .dockerignore to exclude files while copying.
- Test tip-of-tree build within CI.
- Run the test suite outside the Docker image.
- Perform extra sanity tests.
- Switch to make commands for CircleCI.
- Improve Docker README.

docker/Makefile

@@ -1,21 +1,28 @@
-#!/usr/bin/env make
+# A Makefile to build, test, tag and publish the Emscripten SDK Docker container.
 
 # Emscripten version to build: Should match the version that has been already released.
-# i.e.:  1.38.45, 1.38.45-upstream
+# i.e.:  1.39.18
 version =
 alias =
 
+image_name ?= emscripten/emsdk
+
 .TEST:
 ifndef version
 	$(error argument 'version' is not set. Please call `make version=SOME_VERSION ...`)
 endif
 
-build: .TEST
-	docker build --network host --build-arg=EMSCRIPTEN_VERSION=${version} --tag emscripten/emsdk:${version} .
+build: Dockerfile .TEST
+	cd .. && docker build --network host --build-arg=EMSCRIPTEN_VERSION=${version} -t ${image_name}:${version} -f docker/$< .
+
+test: test_dockerimage.sh .TEST
+	# test as non-root
+	docker run --rm -u `id -u`:`id -g` -w /emsdk/docker --net=host ${image_name}:${version} \
+		bash $<
 
 push: .TEST
-	docker push emscripten/emsdk:${version}
+	docker push ${image_name}:${version}
 ifdef alias
-	docker tag emscripten/emsdk:${version} emscripten/emsdk:${alias}
-	docker push emscripten/emsdk:${alias}
+	docker tag ${image_name}:${version} ${image_name}:${alias}
+	docker push ${image_name}:${alias}
 endif