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/test_dockerimage.sh

@@ -1,9 +1,10 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -ex
 
-sudo -u nobody `which emcc` --version
+if [ $EUID -eq 0 ]; then
+  sudo -u nobody `which emcc` --version
+fi
 
-which asm2wasm
 which llvm-ar
 which emsdk
 node --version
@@ -15,5 +16,15 @@ emcc --version
 java -version
 cmake --version
 
-# cleanup after test
-find ${EMSDK} -name "*.pyc" -exec rm {} \;
+exit_code=0
+
+# test emcc compilation
+echo 'int main() { return 0; }' | emcc -o /tmp/main.js -xc -
+node /tmp/main.js || exit_code=$?
+if [ $exit_code -ne 0 ]; then
+  echo "Node exited with non-zero exit code: $exit_code"
+  exit $exit_code
+fi
+
+# test embuilder
+embuilder build zlib --force